MIC2177
2.5A Synchronous Buck Regulator
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (
408
) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
April 2008
M9999-042108
General Description
The Micrel MIC2177 is a 200kHz synchronous buck (step-
down) switching regulator designed for high-efficiency,
battery-powered applications.
The MIC2177 operates from a 4.5V to 16.5V input and
features internal power MOSFETs that can supply up to 2.5A
output current. It can operate with a maximum duty cycle of
100% for use in low-dropout conditions. It also features a
shutdown mode that reduces quiescent current to less than
5µA.
The MIC2177 achieves high efficiency over a wide output
current range by switching between PWM and skip mode.
Operating mode is automatically selected according to output
conditions. Switching frequency is preset to 200kHz and can
be synchronized to an external clock signal of up to 300kHz.
The MIC2177 uses current-mode control with internal current
sensing. Current-mode control provides superior line
regulation and makes the regulator control loop easy to
compensate. The output is protected with pulse-by-pulse
current limiting and thermal shutdown. Undervoltage lockout
turns the output off when the input voltage is less than 4.5V.
The MIC2177 is packaged in a 20-pin wide power SO
package with an operating temperature range of –40°C to
+85°C.
See the MIC2178 for externally selected PWM or skip-mode
operation.
Data sheets and support documentation can be found on
Micrel’s web site at: www.micrel.com.
Features
• 4.5V to 16.5V input voltage range
• Dual-mode operation for high efficiency (up to 96%)
– PWM mode for > 200mA load current
– Skip mode for < 200mA load current
• 100m internal power MOSFETs at 12V input
• 200kHz preset switching frequency
• Low quiescent current
– 1.0mA in PWM mode
– 500µA in skip mode
– < 5µA in shutdown mode
• 100% duty cycle for low dropout operation
• Current-mode control
– Simplified loop compensation
– Superior line regulation
• Current limit
• Thermal shutdown
• Undervoltage lockout
Applications
• High-efficiency, battery-powered supplies
• Buck (step-down) dc-to-dc converters
• Cellular telephones
• Laptop computers
• Hand-held instruments
• Battery Charger
_________________________________________________________________________________________________________
Typical Application
BIASSGND
EN
COMP
PGND
FB
OUT
VIN
C2
100µF
10V
V
OUT
5V/1A
L1, 50µH
C
C
6.8nF
C3 R1
0.01µF 10k
MIC
2177-5.0
AUTO
SYNC
R1
10k
V
IN
5.4V to 18V
C1
22µF
35V U1
20
18
11
13 14–17 19
12
4–7
3,8
1,2,9
D1
MBRS130L
SW
10
ENABLE
SHUTDOWN
2.2
nF
70
75
80
85
90
95
100
10 100 1000 2500
EFFICIENCY (%)
OUTPUT CURRENT (mA)
5V Output
Efficiency
V
IN
=6V
SKIP
PWM
Micrel, Inc. MIC2177
April 2008
2 M9999-042108
Ordering Information
Part Number Output
Voltage Switching
Frequency Temperature Range Package Lead Finish
MIC2177-3.3BWM 3.3V 200kHz –40°C to +85°C 20-Pin Wide SOIC Standard
MIC2177-5.0BWM 5.0V 200kHz –40°C to +85°C 20-Pin Wide SOIC Standard
MIC2177BWM Adj. 200kHz –40°C to +85°C 20-Pin Wide SOIC Standard
MIC2177-3.3YWM 3.3V 200kHz –40°C to +85°C 20-Pin Wide SOIC Pb-Free
Pin Configur ation
2VIN
3SW
4PGND
5PGND
6PGND
7PGND
1VIN
8SW
9
VIN
EN20
BIAS19
SYNC18
SGND17
SGND16
SGND15
10
OUT
14
13
12
11 AUTO
FB
COM
P
SGND
20-Pin Wide SOIC (WM)
Micrel, Inc. MIC2177
April 2008
3 M9999-042108
Pin Description
Pin Number Pin Name Pin Function
1, 2, 9 VIN Supply Input: Controller and switch supply. Unregulated supply input to internal
regulator, output switches, and control circuitry. Requires bypass capacitor to PGND.
All three pins must be connected to VIN.
3, 8 SW Switch (Output): Internal power MOSFET switch output. Both pins must be
externally connected together.
4, 5, 6, 7 PGND Power Ground: Output stage ground connections. Connect all pins to a common
ground plane.
10 OUT
Output Voltage Sense (Input): Senses output voltage to determine minimum switch
current for PWM operation. Connect directly to VOUT.
11 AUTO
Automatic Mode: Connect 2.2nF timing capacitor for automatic PWM-/skip-mode
switching. Regulator operates exclusively in PWM mode when pin is pulled low.
12 FB
Feedback (Input): Error amplifier inverting input. For adjustable output version,
connect FB to external resistive divider to set output voltage. For 3.3V and 5V fixed
output versions, connect FB directly to output.
13 COMP
Compensation: Internal error amplifier output. Connect to capacitor or series RC
network to compensate the regulator control loop.
14, 15, 16, 17 SGND Signal Ground: Ground connection of control section. Connect all pins to common
ground plane.
18 SYNC
Frequency Synchronization (Input): Optional clock input. Connect to external clock
signal to synchronize oscillator. Leading edge of signal above 1.7V terminates
switching cycle. Connect to SGND if not used.
19 BIAS
Bias Supply: Internal 3.3V bias supply output. Decouple with 0.01µF bypass
capacitor and 10k to SGND. Do not apply any external load.
20 EN
Enable (Input): Logic high enables operation. Logic low shuts down regulator. Do
not allow pin to float.
Micrel, Inc. MIC2177
April 2008
4 M9999-042108
Absolute Maximum Ratings
Supply Voltage [100ms transient] (V
IN
) ..........................18V
Output Switch Voltage (V
SW
). .........................................18V
Output Switch Current (I
SW
)...........................................6.0A
Enable, Output-Sense Voltage (V
EN
, V
OUT
). ...................18V
Sync Voltage (V
SYNC
)........................................................6V
Operating Ratings
Supply Voltage (V
IN
)....................................... 4.5V to 16.5V
Junction Temperature (T
J
) ........................–40°C to +125°C
Electrical Characteristics
V
IN
= 7.0V; T
A
= 25°C, bold values indicate –40°C< T
A
< +85°C, unless noted.
Symbol Parameter Condition Min Typ Max Units
PWM mode, output not switching,
4.5V V
IN
16.5V
1.0
1.5 mA
skip mode, output not switching,
4.5V V
IN
16.5V
500
650 µA
I
SS
Input Supply Current
V
EN
= 0V, 4.5V V
IN
16.5V 1 25 µA
V
BIAS
Bias Regulator Output Voltage V
IN
= 16.5V 3.10 3.30 3.40 V
V
FB
Feedback Voltage MIC2177 [adj.]: V
OUT
= 3.3V, I
LOAD
= 0 1.22 1.245 1.27 V
MIC2177 [adj.]: V
OUT
= 3.3V,
5V V
IN
16V, 10mA I
LOAD
2A
3.20
3.14
3.3 3.40
3.46
V
V
MIC2177-5.0: I
LOAD
= 0 4.85 5.0 5.15 V
MIC2177-5.0:
6V V
IN
16V, 10mA I
LOAD
2A
4.85
4.75
5.0 5.15
5.25
V
V
MIC2177-3.3: I
LOAD
= 0 3.20 3.3 3.40 V
V
OUT
Output Voltage
MIC2177-3.3:
5V V
IN
16V, 10mA I
LOAD
2A
3.20
3.14
3.3 3.40
3.46
V
V
V
TH
upper threshold 4.25
4.35 V
V
TL
Undervoltage Lockout
lower threshold 3.9 4.15 V
MIC2177 [adj.] 60 150 nA I
FB
Feedback Bias Current
MIC2177-5.0, MIC2177-3.3 20 40 µA
A
VOL
Error Amplifier Gain 0.6V V
COMP
0.8V 15 18 20 V
upper limit 0.9 1.5 V Error Amplifier Output Swing
Lower limit 0.05 0.1 V
Error Amplifier Output Current source and sink 15 25 35 µA
f
O
Oscillator Frequency 160 200 240 kHz
D
MAX
Maximum Duty Cycle V
FB
= 1.0V 100 %
t
ON min
Minimum On-Time V
FB
= 1.5V 300 400 ns
SYNC Frequency Range 220 300 kHz
SYNC Threshold 0.8 1.6 2.2 V
SYNC Minimum Pulse Width 500 ns
I
SYNC
SYNC Leakage V
SYNC
= 0V to 5.5V –1 0.01 1 µA
PWM mode, V
IN
= 12V 3.8 4.7 5.7 A I
LIM
Current Limit
skip mode 600 mA
high-side switch, V
IN
= 12V 90 250 m R
ON
Switch On-Resistance
low-side switch, V
IN
= 12V 110 250 m
Micrel, Inc. MIC2177
April 2008
5 M9999-042108
Symbol Parameter Condition Min Typ Max Units
I
SW
Output Switch Leakage V
SW
= 16.5V 1 10 µA
Enable Threshold 0.8 1.6
2.2 V
I
EN
Enable Leakage V
EN
= 0V to 5.5V –1 0.01 1 µA
AUTO Threshold 0.8 1.6 V
AUTO Source Current V
FB
= 1.5V, VAUTO < 0.8V 7 11 15 µA
V
IN
– V
OUT
= 0V 220 mA Minimum Switch Current for
PWM Operation V
IN
– V
OUT
= 3V 420 mA
General Note: Devices are ESD sensitive. Handling precautions recommended.
Micrel, Inc. MIC2177
April 2008
6 M9999-042108
Typical Characteristics
175
180
185
190
195
200
205
-60 -30 0 30 60 90 120 150
FREQUENCY (kHz)
TEMPERATURE (°C)
Oscillator Frequenc
y
vs. Temperature
1.238
1.240
1.242
1.244
1.246
1.248
1.250
1.252
REFERENCE VOLTAGE (V)
MIC2177 [adj.]
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Referenc e Voltage
vs. Temperature
3.280
3.285
3.290
3.295
3.300
3.305
3.310
3.315
3.320
REFERENCE VOLTAGE (V)
MIC2177-3.3
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Referenc e Voltage
vs. Temperature
4.970
4.980
4.990
5.000
5.010
5.020
5.030
REFERENCE VOLTAGE (V)
MIC21775.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Referenc e Voltage
vs. Temperature
16.0
16.5
17.0
17.5
18.0
18.5
19.0
AMPLIFIER VOLTAGE GAIN
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Error-Amplifier Gain
vs. Temperature
0
20
40
60
80
100
120
BIAS CURRENT (nA)
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Feedback Input Bias C urrent
vs. Temperature
4.0
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5.0
CURRENT LIMIT (A)
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Current Limit
vs. Temperature
0
50
100
150
200
250
24681012141618
INPUT VOLTAGE (V)
High-Sid e Sw itc
h
On-Resistance
125°C
85°C
25°C
0°C
0
50
100
150
200
250
300
350
24681012141618
INPUT VOLTAGE (V)
Low-Side Switch
On-Resistance
125°C
85°C
25°C
0°C
0
2
4
6
8
10
12
2 4 6 8 10 12 14 16 18
SUPPLY CURRENT (mA)
INPUT VOLTAGE (V)
PWM-Mode
Suppl y Current
OUTPUT
SWITCHING
60
65
70
75
80
85
90
95
100
10 100 1000 2500
EFFICIENCY (%)
OUTPUT CURRENT (mA)
V
IN
=5V
8V
12V
SKIP
PWM
3.3V Output
Efficiency
70
75
80
85
90
95
100
10 100 1000 2500
EFFICIENCY (%)
OUTPUT CURRENT (mA)
8V
12V
V
IN
=6V
SKIP
PWM
5V Output
Efficiency
Micrel, Inc. MIC2177
April 2008
7 M9999-042108
Functional Diagram
V
OUT
= 1.245 R1
R2 + 1
SW
PGND
I
SENSE
Amp.
PWM/
Skip-Mode
Select Logic
V
REF
1.245V
FB
100m
N-channel
100m
P-channel
COMP
VIN
Error
Amp.
Skip-Mode
Comp.
I
MIN
Comp.
I
LIMIT
Comp.
Output
Control
Logic
I
MIN
Thrshld.
Low Output
Comp.
40mV
R
S
Q
200kHz
Oscillator
PWM
Comp.
3.3V
Regulator
UVLO,
Thermal
Shutdown
V
OUT
L1
OUT
SGND
2.2nF
C
C
AUTO
0.01µF
EN
BIAS
V
IN
4.5V to 16.5V
C
IN
SYNC
C
OUT
MIC2177 [Adjustable]
internal
supply
voltage
Enable
Shutdown
Auto-Mode
PWM
20
19
18
11
13
21
3
8
D
4
5
6
7
R1
R2
12
10
14 15 16 17
Bold lines indicate
high current traces
R
C
10k
CORRECTIVE RAMP
RESET PULSE
10µA
3.3V
9
( )
Micrel, Inc. MIC2177
April 2008
8 M9999-042108
Functional Description
Micrel’s MIC2177 is a synchronous buck regulator that
operates from an input voltage of 4.5V to 16.5V and
provides a regulated output voltage of 1.25V to 16.5V. It
has internal power MOSFETs that supply up to 2.5A of
load current and operates with up to 100% duty cycle to
allow low-dropout operation. To optimize efficiency, the
MIC2177 operates in PWM and skip mode. Skip mode
provides the best efficiency when load current is less
than 200mA, while PWM mode is more efficient at higher
current. A patented technique allows the MIC2177 to
automatically select the correct operating mode as the
load current changes.
During PWM operation, the MIC2177 uses current-mode
control which provides superior line regulation and
makes the control loop easier to compensate. The PWM
switching frequency is set internally to 200kHz and can
be synchronized to an external clock frequency up to
300kHz. Other features include a low-current shutdown
mode, current limit, undervoltage lockout, and thermal
shutdown. See the following sections for details.
Switch Output
The switch output (SW) is a half H-bridge consisting of a
high-side P-channel and low-side N-channel power
MOSFET. These MOSFETs have a typical on-resistance
of 100m when the MIC2177 operates from a 12V
supply. Anti-shoot-through circuitry prevents the P-
channel and N-channel from turning on at the same
time.
Current Limit
The MIC2177 uses pulse-by-pulse current limiting to
protect the output. During each switching period, a
current limit comparator detects if the P-channel current
exceeds 4.7A. When it does, the P-channel is turned off
until the next switching period begins.
Undervoltage Lockout
Undervoltage lockout (UVLO) turns off the output when
the input voltage (V
IN
) is too low to provide sufficient gate
drive for the output MOSFETs. It prevents the output
from turning on until V
IN
exceeds 4.3V. Once operating,
the output will not shut off until V
IN
drops below 4.2V.
Thermal Shutdown
Thermal shutdown turns off the output when the
MIC2177 junction temperature exceeds the maximum
value for safe operation. After thermal shutdown occurs,
the output will not turn on until the junction temperature
drops approximately 10°C.
Shutdown Mode
The MIC2177 has a low-current shutdown mode that is
controlled by the enable input (EN). When a logic 0 is
applied to EN, the MIC2177 is in shutdown mode and its
quiescent current drops to less than 5µA.
Internal Bias Regulator
An internal 3.3V regulator provides power to the
MIC2177 control circuits. This internal supply is brought
out to the BIAS pin for bypassing by an external 0.01µF
capacitor. Do not connect any external load to the BIAS
pin. It is not designed to provide an external supply
voltage.
Frequency Synchronization
The MIC2177 operates at a preset switching frequency
of 200kHz. It can be synchronized to a higher frequency
by connecting an external clock to the SYNC pin. The
SYNC pin is a logic level input that synchronizes the
oscillator to the rising edge of an external clock signal. It
has a frequency range of 220kHz–300kHz, and can
operate with a minimum pulse-width of 500ns. If
synchronization is not required, connect SYNC to
ground.
Low-Dropout Operation
Output regulation is maintained in PWM or skip mode
even when the difference between V
IN
and V
OUT
decreases below 1V. As V
IN
– V
OUT
decreases, the duty
cycle increases until it reaches 100%. At this point, the
P-channel is kept on for several cycles at a time, and the
output stays in regulation until V
IN
– V
OUT
falls below the
dropout voltage (dropout voltage = P-channel on
resistance × load current).
PWM-Mode Operation
Refer to “PWM-Mode Functional Diagram” which is a
simplified block diagram of the MIC2177 operating in
PWM mode with its associated waveforms.
When operating in PWM mode, the output P-channel
and N-channel MOSFETs are alternately switched on at
a constant frequency and variable duty cycle. A
switching period begins when the oscillator generates a
reset pulse. This pulse resets the RS latch which turns
on the P-channel and turns off the N-channel. During
this time, inductor current (I
L1
) increases and energy is
stored in the inductor. The current sense amplifier (I
SENSE
Amp) measures the P-channel drain-to-source voltage
and outputs a voltage proportional to I
L1
. The output of
I
SENSE
Amp is added to a saw tooth waveform (corrective
ramp) generated by the oscillator, creating a composite
waveform labeled I
SENSE
on the timing diagram. When
I
SENSE
is greater than the error amplifier output, the PWM
comparator will set the RS latch which turns off the P-
channel and turns on the N-channel. Energy is then
Micrel, Inc. MIC2177
April 2008
9 M9999-042108
discharged from the inductor and I
L1
decreases until the
next switching cycle begins. By varying the P-channel
on-time (duty cycle), the average inductor current is
adjusted to whatever value is required to regulate the
output voltage.
The MIC2177 uses current-mode control to adjust the
duty cycle and regulate the output voltage. Current-
mode control has two signal loops that determine the
duty cycle. One is an outer loop that senses the output
voltage, and the other is a faster inner loop that senses
the inductor current. Signals from these two loops
control the duty cycle in the following way: V
OUT
is fed
back to the error amplifier which compares the feedback
voltage (V
FB
) to an internal reference voltage (V
REF
).
When V
OUT
is lower than its nominal value, the error
amplifier output voltage increases. This voltage then
intersects the current-sense waveform later in switching
period which increases the duty cycle and average
inductor current. If V
OUT
is higher than nominal, the error
amplifier output voltage decreases, reducing the duty
cycle.
The PWM control loop is stabilized in two ways. First,
the inner signal loop is compensated by adding a
corrective ramp to the output of the current sense
amplifier. This allows the regulator to remain stable
when operating at greater than 50% duty cycle. Second,
a series resistor-capacitor load is connected to the error
amplifier output (COMP pin). This places a pole-zero
pair in the regulator control loop.
One more important item is synchronous rectification. As
mentioned earlier, the N-channel output MOSFET is
turned on after the P-channel turns off. When the N-
channel turns on, its on-resistance is low enough to
create a short across the output diode. As a result,
inductor current flows through the N-channel and the
voltage drop across; it is significantly lower than a diode
forward voltage. This reduces power dissipation and
improves efficiency to greater than 95% under certain
operating conditions.
To prevent shoot through current, the output stage
employs break-before-make circuitry that provides
approximately 50ns of delay from the time one MOSFET
turns off and the other turns on. As a result, inductor
current briefly flows through the output diode during this
transition.
Skip-Mode Operation
Refer to “Skip-Mode Functional Diagram” which is a
simplified block diagram of the MIC2177 operating in
skip mode and its associated waveforms.
Skip-mode operation turns on the output P-channel at a
frequency and duty cycle that is a function of V
IN
, V
OUT
,
and the output inductor value. While in skip mode, the N-
channel is kept off to optimize efficiency by reducing
gate charge dissipation. V
OUT
is regulated by skipping
switching cycles that turn on the P-channel.
To begin analyzing MIC2177 skip-mode operation,
assume the skip-mode comparator output is high and
the latch output has been reset to a logic 1. This turns on
the P-channel and causes I
L1
to increase linearly until it
reaches a current limit of 600mA. When I
L1
reaches this
value, the current limit comparator sets the RS latch
output to logic 0, turning off the P-channel. The output
switch voltage (V
SW
) then swings from V
IN
to 0.4V below
ground, and I
L1
flows through the Schottky diode. L1
discharges its energy to the output and I
L1
de-creases to
zero. When I
L1
= 0, V
SW
swings from –0.4V to V
OUT
, and
this triggers a one-shot that resets the RS latch.
Resetting the RS latch turns on the P-channel, which
begins another switching cycle.
The skip-mode comparator regulates V
OUT
by controlling
when the MIC2177 skips cycles. It compares V
FB
to V
REF
and has 10mV of hysteresis to prevent oscillations in the
control loop. When V
FB
is less than V
REF
– 5mV, the
comparator output is logic 1, allowing the P-channel to
turn on. Conversely, when V
FB
is greater than V
REF
+
5mV, the P-channel is turned off.
Note that this is a self-oscillating topology which explains
why the switching frequency and duty cycle are a
function of V
IN
, V
OUT
, and the value of L1. It has the
unique feature (for a pulse-skipping regulator) of
supplying the same value of maximum load current for
any value of V
IN
, V
OUT
, or L1. This allows the MIC2177 to
always supply up to 300mA of load current (I
LOAD
) when
operating in skip mode.
Changing from PWM to Skip Mode
Refer to “Block Diagram” for circuits described in the
following sections.
The MIC2177 automatically changes from PWM to skip
mode operation when I
LOAD
drops below a minimum
value. I
MIN
is determined indirectly by detecting when the
peak inductor current (I
L(peak)
) is less than 420mA. This is
done by the minimum current comparator which detects
if the output P-Channel current equals 420mA during
each switching cycle. If it does not, the PWM/skip-mode
select logic places the MIC2177 into skip-mode
operation.
The value of I
MIN
that corresponds to I
L1(peak)
= 420mA is
given by the following equation:
2
I420mA
I
L1
MIN
−
=
Where:
I
L1
= inductor ripple current
This equation shows I
MIN
varies as a function of I
L
.
Therefore, the user must select an inductor value that
results in I
MIN
= 200mA when I
L(peak)
= 420mA. The
formulas for calculating the correct inductor value are
Micrel, Inc. MIC2177
April 2008 10
M9999-042108
given in the “Applications Information” section. Note that
I
L
varies as a function of input voltage, and this also
causes I
MIN
to vary. In applications where the input
voltage changes by a factor of two, I
MIN
will typically vary
from 130mA to 250mA.
During low-dropout operation, the minimum current
thresh-old circuit reduces the minimum value of I
L1(peak)
for PWM operation. This compensates for I
L1
decreasing to almost zero when the difference between
V
IN
and V
OUT
is very low.
Changing from Skip to PWM Mode
The MIC2177 will automatically change from skip to
PWM mode when I
LOAD
exceeds 300mA. During skip-
mode operation, it can supply up to 300mA, and when
I
LOAD
exceeds this limit, V
OUT
will fall below its nominal
value. At this point, the MIC2177 begins operating in
PWM mode. Note that the maximum value of I
LOAD
for
skip mode is greater than the minimum value required
for PWM mode. This current hysteresis prevents the
MIC2177 from toggling between modes when I
LOAD
is in
the range of 100mA to 300mA.
The low output comparator determines when V
OUT
is low
enough for the regulator to change operating modes. It
detects when the feedback voltage is 3% below nominal,
and pulls the AUTO pin to ground. When AUTO is less
than 1.6V, the PWM/skip-mode select logic places the
MIC2177 into PWM operation. The external 2.2nF
capacitor connected to AUTO is charged by a 10µA
current source after the regulator begins operating in
PWM mode. As a result, AUTO stays below 1.6V for
several switching cycles after PWM operation begins,
forcing the MIC2177 to remain in PWM mode during this
transition.
External PWM-Mode Selection
The MIC2177 can be forced to operate in only PWM
mode by connecting AUTO to ground. This prevents
skip-mode operation in applications that are sensitive to
switching noise.
Micrel, Inc. MIC2177
April 2008 11
M9999-042108
PWM-Mode Functional Diag ram
SW
PGND
I
SENSE
Amp.
V
REF
1.245V
100m
N-channel
100m
P-channel
COMP
VIN
Error
Amp.
R
S
Q
200kHz
Oscillator
PWM
Comp.
V
OUT
L1
FB
SGND
C
C
V
IN
4.5V to 16.5V
C
IN
SYNC
C
OUT
MIC2177 [Adjustable] PWM-Mode Signal Path
Stop
18
13
2
1
3
8
D
4
5
6
7
R1
R2
12
14 15 16 17
R
C
Corrective
Ramp
Reset
Pulse
I
L1
9
V
OUT
= 1.245 R1
R2 + 1
( )
VSW
IL1
Reset
Pulse
ISENSE
ILOAD
IL1
Error Amp.
Output
Micrel, Inc. MIC2177
April 2008 12
M9999-042108
Skip-Mode Functional Diagram
S
R
Q
One
Shot SW
PGND
I
SENSE
Amp.
V
REF
1.245V
100m
P-channel
VIN
Skip-Mode
Comp.
I
LIMIT
Comp.
V
OUT
L1
FB
SGND
V
IN
4.5V to 16.5V
C
IN
C
OUT
MIC2177 [ Adjustabl e] Skip-Mod e Signal P at
21
3
8
D
4
5
6
7
R1
R2
12
14 15 16 17
I
LIMIT
Thresh.
Voltage
Output Control Logic
I
L1
9
V
OUT
= 1.245 R1
R2 + 1
( )
V
SW
I
L1
One-Shot
Pulse
V
FB
V
REF
+ 5mV
V
REF
– 5mV
0
I
LIM
0
V
OUT
V
IN
Micrel, Inc. MIC2177
April 2008 13
M9999-042108
Application Information
Feedback Resistor Selection (Adjustable Version)
The output voltage is configured by connecting an
external resistive divider to the FB pin as shown in
“MIC2177 Block Diagram.” The ratio of R1 to R2
determines the output voltage. To optimize efficiency
during low output current operation, R2 should not be
less than 20k. However, to prevent feedback error due
to input bias current at the FB pin, R2 should not be
greater than 100k. After selecting R2, calculate R1
using the following formula:
⎥
⎦
⎤
⎢
⎣
⎡−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=1
1.245V
V
R2R1
OUT
Input Capacitor Selection
The input capacitor is selected for its RMS current and
voltage rating and should be a low ESR (equivalent
series resistance) electrolytic or tantalum capacitor. As a
rule-of-thumb, the voltage rating for a tantalum capacitor
should be twice the value of V
IN
, and the voltage rating
for an electrolytic should be 40% higher than V
IN
. The
RMS current rating must be equal or greater than the
maximum RMS input ripple current. A simple, worst-case
formula for calculating this RMS current is:
2
I
I
LOAD(max)
RMS(max)
=
Tantalum capacitors are a better choice for applications
that require the most compact layout or operation below
0°C. The input capacitor must be located very close to
the V
IN
pin (within 0.2 inches, 5mm). Also place a 0.1µF
ceramic bypass capacitor as close as possible to V
IN
.
Inductor Selection
The inductor must be at least a minimum value in order
for the MIC2177 to change from PWM to skip mode at
the correct value of output current. This minimum value
ensures the inductor ripple current never exceeds
600mA, and is calculated using the following formula:
8.3µ.3µ
V
V
1VL
IN(max)
OUT
OUTMIN
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛−=
Where:
V
IN(max)
= maximum input voltage
In general, a value at least 20% greater than L
MIN
should
be selected because inductor values have a tolerance of
±20%.
Two other parameters to consider in selecting an
inductor are winding resistance and peak current rating.
The inductor must have a peak current rating equal or
greater than the peak inductor current. Otherwise, the
inductor may saturate, causing excessive current in the
output switch. Also, the inductor’s core loss may
increase significantly. Both of these effects will degrade
efficiency. The formula for peak inducto rcurrent is:
I
L(peak)
= I
LOAD(max)
+ 300mA
To maximize efficiency, the inductor’s resistance must
be less than the output switch on-resistance (preferably
50mor less).
Output Capacitor Selection
Select an output capacitor that has a low value of ESR.
This parameter determines a regulator’s output ripple
voltage (V
RIPPLE
) which is generated by I
L
× ESR. As
mentioned in “Inductor Selection,” the maximum value
for I
L
is 600mA.
Therefore, the maximum value of ESR is:
RIPPLE
MAX
V
600mA
ESR =
Where:
V
RIPPLE
< 1% of V
OUT
Typically, capacitors in the range of 100µF to 220µF
have ESR less than this maximum value. The output
capacitor can be either a low ESR electrolytic or
tantalum capacitor, but tantalum is a better choice for
compact layout and operation at temperatures below
0°C. The voltage rating of a tantalum capacitor must be
2 × V
OUT
, and the voltage rating of an electrolytic must
be 1.4 × V
OUT
.
Output Diode Selection
In PWM operation, inductor current flows through the
output diode approximately 50ns during the dead time
when one output MOSFET turns off and the other turns
on. In skip-mode, the inductor current flows through the
diode during the entire P-channel off time. The correct
diode for both of these conditions is a 1A diode with a
reverse voltage rating greater than V
IN
. It must be a
Schottky or ultra fast-recovery diode (t
R
<100ns) to
minimize power dissipation from the diode’s reverse-
recovery charge.
Compensation
Compensation is provided by connecting a series RC
load to the COMP pin. This creates a pole-zero pair in
the regulator control loop, allowing the regulator to
remain stable with enough low frequency loop-gain for
good load and line regulation. At higher frequencies
pole-zero reduces loop-gain to a level referred to as the
mid-band gain. The mid-band gain is low enough so that
the loop gain crosses 0dB with sufficient phase margin.
Typical values for the RC load are 4.7nF – 10nF for the
capacitor and 5k – 20k for the resistor.
Printed Circuit Board Layout
A well designed PC board will prevent switching noise
and ground bounce from interfering with the operation of
Micrel, Inc. MIC2177
April 2008
14 M9999-042108
the MIC2177. A good design takes into consideration
component placement and routing of power traces.
The first thing to consider is the locations of the input
capacitor, inductor, output diode, and output capacitor.
The input capacitor must be placed very close to the V
IN
pin, the inductor and output diode very close to the SW
pin, and the output capacitor near the inductor. These
components pass large high-frequency current pulses,
so they must use short, wide power traces. In addition,
their ground pins and PGND are connected to a ground
plane that is nearest the power supply ground bus.
The feedback resistors, RC compensation network, and
BIAS pin bypass capacitor should be located near their
respective pins. To prevent ground bounce, their ground
traces and SGND should not be in the path of switching
currents returning to the power supply ground bus.
SGND and PGND should be tied together by a ground
plane that extends under the MIC2177.
BIASSGND
AUTO
COMP
PGND
FB
SW
VIN
C2
100µF
10V
VOUT
3.3V/1A
L1, 50µH
C4
6.8nF C3
0.01µF
MIC2177
SYNC
EN
R4
10k R4
10k
VIN
4
.5V to 16.5V
C1
22µF
35V
U1
20
18
10
11
13 14–17 19
12
4–7
3,8
1,2,9
D1
MBRS130L
U1 Micrel MIC2177-3.3BWM
C1 AVX
C2 AVX
C3 Z5UorX7R Ceramic Dielectric Material
C4 X7RorNP0 Ceramic Dielectric Material
D1 Motorola MBRS130LT3
L1 Coiltronics CTX50-4P, DCR = 0.097
L1 Coilcraft
L1 Bi HM77-11003, DCR = 0.073
C5
0.01
µF
OUT
Bill of Materials
Inductors Capacitors Diodes Transistors
Coilcraft
1102 Silver Lake Rd.
Cary, IL 60013
Tel: (708) 639-2361
Fax: (708) 639-1469
AVX
801 17th Ave.
Myrtle Beach, SC 29577
Tel: (803) 448-9411
Fax: (803) 448-1973
General Instruments (GI)
10 Melville Park Rd.
Melville, NY 11747
Tel: (516) 847-3222
Fax: (516) 847-3150
Siliconix
2201 Laurelwood Rd.
Santa Clara, CA 96056
Tel: (800) 554-5565
Coiltronics
6000 Park of Commerce Blvd.
Boca Raton, FL 33487
Tel: (407) 241-7876
Fax: (407) 241-9339
Sanyo Video Components Corp.
2001 Sanyo Ave.
San Diego, CA 92173
Tel: (619) 661-6835
Fax: (619) 661-1055
International Rectifier Corp.
233 Kansas St.
El Segundo, CA 90245
Tel: (310) 322-3331
Fax: (310) 322-3332
Bi Technologies
4200 Bonita Place
Fullerton, CA 92835
Tel: (714) 447-2345
Fax: (714) 447-2500
Sprague Electric
60005 Lower Main St.
Sanford, ME 04073
Tel: (207) 324-4140
Motorola, Inc.
MS 56-126
3102 North 56th St.
Phoenix, AZ 85018
Tel: (602) 244-3576
Fax: (602) 244-4015
Micrel, Inc. MIC2177
April 2008
15 M9999-042108
Package Information
20-Pin Wide SOIC (WM)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 1999 Micrel, Incorporated.
