Datasheet MIC2177-3.3BWM, MIC2177-5.0BWM, MIC2177BWM Datasheet (MICREL)

MIC2177 MicrelMIC2177 Micrel

70

75

80

85

90

95

100

10 100 1000 2500

EFFICIENCY (%)

OUTPUT CURRENT (mA)

5V Output
Efficiency

VIN = 6V

SKIP

PWM

MIC2177

2.5A Synchronous Buck Regulator

General Description

The Micrel MIC2177 is a 200kHz synchronous buck (step­down) 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-lead 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.

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

• 100mΩ 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

V

IN

6V to 16.5V

C1

22µF

35V

April 1999 1 MIC2177

22µF

AUTO

MODE

C2

35V

PWM
MODE

2.2nF

Mode

C6

OFF

U1

ON

ENABLE

20

EN

11

AUTO

18

SYNC

COMP

R7

15k
C5

10nF

1,2,9

VIN

OUT

MIC2177-5.0

13 14–17 19

SW

PGND

FB

BIASSGND

10

3,8

4–7

12

C4

0.01µF

L1, 33µH

D1
MBRS140

C7
220µF
10V

C3
220µF
10V

V

OUT

5V/2.5A

MIC2177 Micrel

Ordering Information

Part Output Switching Temperature
Number Voltage Frequency Range Package

MIC2177-3.3BWM 3.3V 200kHz –40°C to +85°C 20-lead wide SOP
MIC2177-5.0BWM 5.0V 200kHz –40°C to +85°C 20-lead wide SOP
MIC2177BWM adj. 200kHz –40 °C to +85°C 20-lead wide SOP

Pin Configuration

VIN

OUT

1VIN
2VIN
3SW
4PGND
5PGND
6PGND
7PGND
8SW
9

10

14
13
12
11

EN20
BIAS19
SYNC18
SGND17
SGND16
SGND15
SGND
COMP
FB
AUTO

20-Lead Wide SOP

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 V

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 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.

OUT

.

MIC2177 2 April 1999

MIC2177 Micrel

Absolute Maximum Ratings

Supply Voltage [100ms transient] (VIN).........................18V

Output Switch Voltage (VSW) ........................................18V

Operating Ratings

Supply Voltage (VIN) ..................................... 4.5V to 16.5V

Junction Temperature Range (TJ) ........... –40°C to +125°C

Output Switch Current (ISW).........................................6.0A

Enable, Output-Sense Voltage (VEN, V
Sync Voltage (V

) .....................................................6V

SYNC

) .................18V

OUT

Electrical Characteristics

VIN = 7.0V; TA = 25°C, bold indicates –40°C ≤ TA ≤ 85°C; unless noted.
Symbol Parameter Condition Min Typ Max Units

I

SS

V

BIAS

V

FB

V

OUT

V

TH

V

TL

I

FB

A

VOL

f

O

D

MAX

t

ON min

I

SYNC

I

LIM

R

ON

I

SW

Input Supply Current PWM mode, output not switching, 1.0 1.5 mA

4.5V ≤ VIN ≤ 16.5V
skip mode, output not switching, 500 650 µA

4.5V ≤ VIN ≤ 16.5V

VEN = 0V, 4.5V ≤ VIN ≤ 16.5V 1 25 µA
Bias Regulator Output Voltage VIN = 16.5V 3.10 3.30 3.4 V
Feedback Voltage MIC2177 [adj.]: V
Output Voltage MIC2177 [adj.]: V

5V ≤ VIN ≤ 16V, 10mA ≤ I

MIC2177-5.0: I

= 3.3V, I

OUT

= 3.3V, 3.20 3.3 3.40 V

OUT

LOAD

= 0 4.85 5.0 5.15 V

LOAD

= 0 1.22 1.245 1.27 V

LOAD

≤ 2A 3.14 3.46 V

MIC2177-5.0: 4.85 5.0 5.15

6V ≤ VIN ≤ 16V, 10mA ≤ I

MIC2177-3.3: I

= 0 3.20 3.3 3.40 V

LOAD

≤ 2A 4.75 5.25 V

LOAD

MIC2177-3.3: 3.20 3.3 3.40 V

5V ≤ VIN ≤ 16V, 10mA ≤ I

≤ 2A 3.14 3.46 V

LOAD

Undervoltage Lockout upper threshold 4.25 4.35 V

lower threshold 3.9 4.15 V
Feedback Bias Current MIC2177 [adj.] 60 150 nA

MIC2177-5.0, MIC2177-3.3 20 40 µA
Error Amplifier Gain 0.6V ≤ V

≤ 0.8V 15 18 20

COMP

Error Amplifier Output Swing upper limit 0.9 1.5 V

lower limit 0.05 0.1 V
Error Amplifier Output Current source and sink 15 25 35 µA
Oscillator Frequency 160 200 240 kHz
Maximum Duty Cycle VFB = 1.0V 100 %
Minimum On-Time VFB = 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
SYNC Leakage V

= 0V to 5.5V –1 0.01 1 µA

SYNC

Current Limit PWM mode, VIN = 12V 3.8 4.7 5.7 A

skip mode 600 mA
Switch On-Resistance high-side switch, VIN = 12V 90 250 mΩ

low-side switch, VIN = 12V 110 250 mΩ
Output Switch Leakage VSW = 16.5V 1 10 µA

April 1999 3 MIC2177

MIC2177 Micrel

Symbol Parameter Condition Min Typ Max Units

Enable Threshold 0.8 1.6 2.2 V

I

EN

General Note: Devices are ESD sensitive. Handling precautions recommended.

Enable Leakage VEN = 0V to 5.5V –1 0.01 1 µA
AUTO Threshold 0.8 1.6 V
AUTO Source Current VFB = 1.5V, V
Minimum Switch Current VIN – V

for PWM Operation

VIN – V

= 0V 220 mA

OUT

= 3V 420 mA

OUT

< 0.8V 7 11 15 µA

AUTO

MIC2177 4 April 1999

MIC2177 Micrel

1.238

1.240

1.242

1.244

1.246

1.248

1.250

1.252

-60 -30 0 30 60 90 120 150

REFERENCE VOLTAGE (V)

TEMPERATURE (°C)

16.0

16.5

17.0

17.5

18.0

18.5

19.0

-60 -30 0 30 60 90 120 150

AMPLIFIER VOLTAGE GAIN

TEMPERATURE (°C)

0

50

100

150

200

250

24681012141618

ON-RESISTANCE (mΩ)

INPUT VOLTAGE (V)

70

75

80

85

90

95

100

10 100 1000 2500

EFFICIENCY (%)

OUTPUT CURRENT (mA)

8V

12V

5V Output
Efficiency

VIN = 6V

SKIP

PWM

Typical Characteristics

Oscillator Frequency

205

200

195

190

185

FREQUENCY (kHz)

180

175

vs. Temperature

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

Reference Voltage

5.030

5.020

5.010

5.000

4.990

4.980

REFERENCE VOLTAGE (V)

4.970

vs. Temperature

MIC2177-5.0

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

Reference Voltage

vs. Temperature

MIC2177 [adj.]

Error-Amplifier Gain

vs. Temperature

Reference Voltage

3.320

3.315

3.310

3.305

3.300

3.295

3.290

3.285

REFERENCE VOLTAGE (V)

3.280

vs. Temperature

MIC2177-3.3

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

Feedback Input Bias Current

120

100

BIAS CURRENT (nA)

vs. Temperature

80

60

40

20

0

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

April 1999 5 MIC2177

Current Limit

5.0

4.9

4.8

4.7

4.6

4.5

4.4

4.3

CURRENT LIMIT (A)

4.2

4.1

4.0

vs. Temperature

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

PWM-Mode

12

10

SUPPLY CURRENT (mA)

Supply Current

OUTPUT

SWITCHING

8

6

4

2

0

24681012141618

INPUT VOLTAGE (V)

High-Side Switch

On-Resistance

125°C

3.3V Output

100

95
90
85
80
75

EFFICIENCY (%)

70
65
60

10 100 1000 2500

Efficiency

VIN = 5V

8V

12V

SKIP

PWM

OUTPUT CURRENT (mA)

85°C
25°C

0°C

Low-Side Switch

On-Resistance

125°C

0

24681012141618

INPUT VOLTAGE (V)

ON-RESISTANCE (mΩ)

350
300
250
200
150
100

50

85°C
25°C

0°C

MIC2177 Micrel

Block Diagram

V

IN

4.5V to 16.5V

C

IN

VIN

21

9

Enable

Shutdown

Auto-Mode

PWM

0.01µF

2.2nF

EN

BIAS

SYNC

AUTO

UVLO,

Thermal

Shutdown

Output

Control

Logic

I

SENSE

Amp.

100mΩ

P-channel

SW

3
8

V

OUT

L1

D

20

19

18

3.3V

Regulator

internal
supply
voltage

200kHz

Oscillator

PWM/

Skip-Mode

Select Logic

Comp.

Comp.

CORRECTIVE RAMP

3.3V

I

LIMIT

I

MIN

Thrshld.

Low Output
Comp.

I

MIN

10µA

11

Skip-Mode
Comp.

100mΩ

N-channel

40mV

PGND

4
5
6
7

Bold lines indicate
high current traces

OUT

10

FB

12

1.245

C

OUT

R1

R2

R1

R2

1

V

OUT

RESET PULSE

R

R

C

C

COMP

13

C

MIC2177 [Adjustable]

Q

S

PWM

Comp.

SGND

Error
Amp.

14 15 16 17

V

REF

1.245V

MIC2177 6 April 1999

MIC2177 Micrel

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 high­side P-channel and low-side N-channel power MOSFET.
These MOSFETs have a typical on-resistance of 100mΩ
when the MIC2177 operates from a 12V supply. Antishoot­through 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 (VIN) is too low to provide sufficient gate drive for
the output MOSFETs. It prevents the output from turning on
until VIN exceeds 4.3V. Once operating, the output will not
shut off until VIN 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 VIN and V
1V. As VIN – V

decreases, the duty cycle increases until

OUT

decreases below

OUT

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 VIN – V

falls below the dropout voltage (dropout

OUT

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 N­channel 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 (IL1) increases
and energy is stored in the inductor. The current sense
amplifier (I

SENSE

Amp) measures the P-channel drain-to­source voltage and outputs a voltage proportional to IL1. The
output of I

SENSE

Amp is added to a sawtooth waveform
(corrective ramp) generated by the oscillator, creating a
composite waveform labeled I
When I

is greater than the error amplifier output, the

SENSE

on the timing diagram.

SENSE

PWM comparator will set the RS latch which turns off the P­channel and turns on the N-channel. Energy is then dis­charged from the inductor and I

decreases until the next

L1

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

is fed back to the error amplifier which compares

OUT

the feedback voltage (VFB) to an internal reference voltage
(V

). When V

REF

is lower than its nominal value, the error

OUT

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

is higher than nominal, the error amplifier output

OUT

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

April 1999 7 MIC2177

MIC2177 Micrel

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 VIN, 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

is regulated by skipping switching cycles

OUT

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 IL1 to increase linearly until it reaches a current limit
of 600mA. When IL1 reaches this value, the current limit
comparator sets the RS latch output to logic 0, turning off the
P-channel. The output switch voltage (VSW) then swings from
VIN to 0.4V below ground, and IL1 flows through the Schottky
diode. L1 discharges its energy to the output and IL1 de­creases to zero. When IL1 = 0, VSW swings from –0.4V to
V

, and this triggers a one-shot that resets the RS latch.

OUT

Resetting the RS latch turns on the P-channel, which begins
another switching cycle.

The skip-mode comparator regulates V
when the MIC2177 skips cycles. It compares VFB to V

by controlling

OUT

REF

and
has 10mV of hysteresis to prevent oscillations in the control
loop. When V

is less than V

FB

– 5mV, the comparator

REF

output is logic 1, allowing the P-channel to turn on. Con­versely, when V

is greater than V

FB

+ 5mV, the P-channel

REF

is turned off.
Note that this is a self-oscillating topology which explains

why the switching frequency and duty cycle are a function of
VIN, V

, and the value of L1. It has the unique feature (for

OUT

a pulse-skipping regulator) of supplying the same value of
maximum load current for any value of VIN, V

, or L1. This

OUT

allows the MIC2177 to always supply up to 300mA of load
current (I

) when operating in skip mode.

LOAD

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

drops below a minimum value. I

LOAD

MIN

is
determined indirectly by detecting when the peak inductor
current (I

) is less than 420mA. This is done by the

L(peak)

minimum current comparator which detects if the output P­Channel 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

that corresponds to I

MIN

L1(peak)

= 420mA is

given by the following equation:

I

MIN

420mA I

=

2

−∆

L1

Where:

∆IL1 = inductor ripple current

This equation shows I

varies as a function of ∆IL . There-

MIN

fore, the user must select an inductor value that results in
I

= 200mA when I

MIN

L(peak)

= 420mA. The formulas for cal­culating the correct inductor value are given in the “Applica­tions Information” section. Note that ∆IL varies as a function
of input voltage, and this also causes I
tions where the input voltage changes by a factor of two, I

to vary. In applica-

MIN

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 ∆IL1 decreasing to almost
zero when the difference between VIN and V

is very low.

OUT

Changing from Skip to PWM Mode

The MIC2177 will automatically change from skip to PWM
mode when I
tion, it can supply up to 300mA, and when I
limit, V

OUT

exceeds 300mA. During skip-mode opera-

LOAD

exceeds this

LOAD

will fall below its nominal value. At this point, the
MIC2177 begins operating in PWM mode. Note that the
maximum value of I

for skip mode is greater than the

LOAD

minimum value required for PWM mode. This current hyster­esis prevents the MIC2177 from toggling between modes
when I

The low output comparator determines when V

is in the range of 100mA to 300mA.

LOAD

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.

MIC2177 8 April 1999

MIC2177 Micrel

PWM-Mode Functional Diagram

V

IN

4.5V to 16.5V

C

IN

VIN

2

9

1

Stop

1.245

L1

C

OUT

R1

R2

R1

1

V

OUT

SYNC

I

SENSE

Amp.

100mΩ

P-channel

SW

3
8

V

OUT

L1

I

D

100mΩ

N-channel

Corrective

200kHz

Oscillator

18

Ramp

Reset
Pulse

PGND

4
5
6
7

FB

12

R2

R

Q

S

PWM

Comp.

Error
Amp.

COMP

13

R

C

C

C

V

1.245V

REF

MIC2177 [Adjustable] PWM-Mode Signal Path

14 15 16 17

SGND

V

SW

Reset
Pulse

I

L1

SENSE

LOAD

∆I

L1

I

Error Amp.
Output

I

April 1999 9 MIC2177

MIC2177 Micrel

Skip-Mode Functional Diagram

V

IN

4.5V to 16.5V

C

IN

21

9

1.245

I

L1

C

OUT

R1
R2

1

V

OUT

SW

3
8

V

OUT

L1

D

PGND

4
5
6
7

Output Control Logic

One
Shot

I

LIMIT

Comp.

I

LIMIT

Thresh.
Voltage

VIN

S

Q

R

I

SENSE

100mΩ

P-channel

Amp.

MIC2177 [Adjustable] Skip-Mode Signal Path

V

IN

V

OUT

V

SW

0

One-Shot
Pulse

I

LIM

I

L1

0

V

+ 5mV

REF

V

FB

V

– 5mV

REF

Skip-Mode
Comp.

V

REF

SGND

R1

FB

12

R2

1.245V

14 15 16 17

MIC2177 10 April 1999

MIC2177 Micrel

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 20kΩ. However, to
prevent feedback error due to input bias current at the FB pin,
R2 should not be greater than 100kΩ. After selecting R2,
calculate R1 using the following formula:



V



R1 = R2







1.245V



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-of­thumb, the voltage rating for a tantalum capacitor should be
twice the value of VIN, and the voltage rating for an electrolytic
should be 40% higher than V
be equal or greater than the maximum RMS input ripple
current. A simple, worst-case formula for calculating this
RMS current is:

I

I =

RMS(max)

LOAD(max)

2

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 VIN pin
(within 0.2 inches, 5mm). Also place a 0.1µF ceramic bypass
capacitor as close as possible to VIN.

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:



L = V 1 –

MIN

OUT




Where:

V

= maximum input voltage

IN(max)

In general, a value at least 20% greater than L
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 inductor
current is:

I I 300mA

L(peak)

=+

LOAD(max)





−

1









The RMS current rating must

IN.

OUT



8.3 H/V

×µ




MIN

V

V

IN(max)

should be

To maximize efficiency, the inductor’s resistance must be
less than the output switch on-resistance (preferably 50mΩ
or 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

) which is generated by ∆IL × ESR. As mentioned in

RIPPLE

“Inductor Selection,” the maximum value for ∆IL is 600mA.
Therefore, the maximum value of ESR is:

ESR =

MAX

600mA

V

RIPPLE

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
electrolytic must be 1.4 × V

, and the voltage rating of an

OUT

.

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 VIN. It must be a Schottky or ultrafast-recovery diode
(tR< 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 loop­gain 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 5kΩ – 20kΩ 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 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 VIN 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.

April 1999 11 MIC2177

MIC2177 Micrel

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

V

IN

4.5V to 16.5V
C1

22µF
35V

C5

0.01
µF

20

18

11

6.8nF

U1

EN
SYNC

AUTO

COMP
R4

10k

C4

1,2,9

VIN

MIC2177

13 14–17

Figure 1. MIC2177 4.5V–16.5V to 3.3/1A Regulator

currents returning to the power supply ground bus. SGND
and PGND should be tied together by a ground plane that
extends under the MIC2177.

10

OUT

SW

PGND

FB

BIASSGND

19

C3

0.01µF

L1, 50µH

3,8

D1
MBRS130L

4–7

12

U1 Micrel MIC2177-3.3BWM
C1 AVX TPSE226M035R0300, ESR = 0.3Ω
C2 AVX TPSD107M010R0100, ESR = 0.1Ω
C3 Z5UorX7R Ceramic Dielectric Material
C4 X7RorNP0 Ceramic Dielectric Material
D1 Motorola MBRS130LT3
L1 Coiltronics CTX50-4P, DCR = 0.097Ω
L1 Coilcraft DO3316P-473, DCR = 0.12Ω
L1 Bi HM77-11003, DCR = 0.073Ω

C2
100µF
10V

V

OUT

3.3V/1A

Suggested Manufacturers List

Inductors Capacitors Diodes Transistors

Coilcraft AVX Corp. General Instruments (GI) Siliconix

1102 Silver Lake Rd. 801 17th Ave. South 10 Melville Park Rd. 2201 Laurelwood Rd.
Cary, IL 60013 Myrtle Beach, SC 29577 Melville, NY 11747 Santa Clara, CA 96056
tel: [phone] tel: [phone] tel: [phone] tel: [phone]
fax: [phone] fax: [phone] fax: [phone]

Coiltronics Sanyo Video Components Corp. International Rectifier Corp.

6000 Park of Commerce Blvd. 2001 Sanyo Ave. 233 Kansas St.
Boca Raton, FL 33487 San Diego, CA 92173 El Segundo, CA 90245
tel: [phone] tel: [phone] tel: [phone]
fax: [phone] fax: [phone] fax: [phone]

Bi Technologies Sprague Electric Motorola Inc.

4200 Bonita Place Lower Main St. MS 56-126
Fullerton, CA 60005 Sanford, ME 04073 3102 North 56th St.
tel: [phone] tel: [phone] Phoenix, AZ 85018
fax: [phone] tel: [phone]

fax: [phone]

MIC2177 12 April 1999

MIC2177 Micrel

Package Information

PIN 1

DIMENSIONS:

0.301 (7.645)

0.297 (7.544)

INCHES (MM)

0.027 (0.686)

0.031 (0.787)

0.094 (2.388)

0.090 (2.286)

0.050 (1.270)
TYP

0.509 (12.929)

0.505 (12.827)

0.016 (0.046)
TYP

0.103 (2.616)

0.099 (2.515)

SEATING

PLANE

0.015

(0.381)

0.015

(0.381)

MIN

TYP
R

7°

20-Lead Wide SOP (WM)

0.297 (7.544)

0.293 (7.442)

0.330 (8.382)

0.326 (8.280)

0.032 (0.813) TYP

0.408 (10.363)

0.404 (10.262)

0.022 (0.559)

0.018 (0.457)

10° TYP

5°
TYP

April 1999 13 MIC2177

MIC2177 Micrel

MIC2177 14 April 1999

MIC2177 Micrel

April 1999 15 MIC2177

MIC2177 Micrel

MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 [phone] FAX + 1 [phone] WEB http://www.micrel.com

This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or

other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.

© 1999 Micrel Incorporated

MIC2177 16 April 1999