Datasheet MIC2179-3.3BSM, MIC2179-5.0BSM, MIC2179BSM Datasheet (MICREL)

MIC2179

Micrel

MIC2179

1.5A Synchronous Buck Regulator

General Description

The Micrel MIC2179 is a 200kHz synchronous buck (step­down) switching regulator designed for high-efficiency, bat­tery-powered applications.

The MIC2179 operates from a 4.5V to 16.5V input and
features internal power MOSFETs that can supply up to 1.5A
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 MIC2179 achieves high efficiency over a wide output
current range by operating in either PWM or skip mode. The
operating mode is externally selected, typically by an intelli­gent system, which chooses the appropriate mode according
to operating conditions, efficiency, and noise requirements.
The switching frequency is preset to 200kHz and can be
synchronized to an external clock signal of up to 300kHz.

The MIC2179 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 MIC2179 and is packaged in a 20-lead SSOP package
with an operating temperature range of –40°C to +85°C.

Features

• 4.5V to 16.5V input voltage range

• Dual-mode operation for high efficiency (up to 96%)
PWM mode for > 150mA load current
Skip mode for <150mA load current

• 150mΩ internal power MOSFETs at 12V input

• 200kHz preset switching frequency

• Low quiescent current

1.0mA in PWM mode
600µA in skip mode
< 5µA in shutdown mode

• Current-mode control
Simplified loop compensation
Superior line regulation

• 100% duty cycle for low dropout operation

• 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

5.4V to 16.5V
C1

10µF

20V

Output Good

Output Low

Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 [phone] • fax + 1 [phone] • http://www.micrel.com

Skip Mode

PWM Mode

R1
20k

15

13

C4

6.8nF

U1

EN

6

PWRGD

5

PWM
SYNC

COMP

R2

10k

June 1998 1 MIC2179

8

16,17

VIN

MIC

2179-3.3

9–12 14

SW

PGND

FB

BIASSGND

3,4

1,2,
19,20

7

C3

0.01µF

L1

22µH

D1
MBRM120

Pins 4 and 18 are not connected.
Pins 3 and 4 can be connected

together for a low-impedance
connection.

C2
100µF

6.3V

V

OUT

3.3V/600mA

MIC2179

Ordering Information

Part Number Voltage Temperature Range Package

MIC2179BSM Adjustable –40°C to +85°C 20-lead SSOP
MIC2179-3.3BSM 3.3V –40°C to +85°C 20-lead SSOP
MIC2179-5.0BSM 5.0V –40°C to +85°C 20-lead SSOP

Pin Configuration

Micrel

PGND

SW

NC

PWM

PWRGD

FB

COMP

SGND
SGND

Pin Description

Pin Number Pin Name Pin Function

1, 2, 19, 20 PGND Power Ground: Connect all pins to central ground point.

3 SW Switch (Output): Internal power MOSFET output switches.
5 PWM PWM/Skip-Mode Control (Input): Logic-level input. Controls regulator

6 PWRGD Error Flag (Output): Open-drain output. Active low when FB input is 10%

7 FB Feedback (Input): Connect to output voltage divider resistors.
8 COMP Compensation: Output of internal error amplifier. Connect capacitor or

9–12 SGND Signal Ground: Connect all pins to ground, PGND.

13 SYNC Frequency Synchronization (Input): Optional. Connect an external clock

14 BIAS Internal 3.3V Bias Supply: Decouple with 0.01µF bypass capacitor to

15 EN Enable (Input): Logic high enables operation. Logic low shuts down

16, 17 VIN Supply Voltage (Input): Requires bypass capacitor to PGND. Both pins

4, 18 NC not internally connected.

1PGND
2
3
4
5
6
7
8
9

10

19
18
17
16
15
14
13
12
11

PGND20
PGND
NC
VIN
VIN
EN
BIAS
SYNC
SGND
SGND

20-Lead Wide SSOP

operating mode. Logic low enables PWM mode. Logic high enables skip
mode. Do not allow pin to float.

below the reference voltage (V

series RC network to compensate the regulator control loop.

signal to synchronize the oscillator. Leading edge of signal above 1.7V
terminates switching cycle. Connect to SGND if not used.

SGND. Do not apply any external load.

regulator. Do not allow pin to float.

must be connected to VIN.

REF

).

MIC2179 2 June 1998

MIC2179

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, PWM Control Voltage (VEN, V
Sync Voltage (V

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

SYNC

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

PWM

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, 600 750 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 MIC2179 [adj.]: V
Output Voltage MIC2179 [adj.]: V

5V ≤ VIN ≤ 16V, 10mA ≤ I

MIC2179-5.0: 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

≤ 1A 3.14 3.46 V

MIC2179-5.0: 4.85 5.0 5.15

6V ≤ VIN ≤ 16V, 10mA ≤ I

MIC2179-3.3: I

= 0 3.20 3.3 3.40 V

LOAD

≤ 1A 4.75 5.25 V

LOAD

MIC2179-3.3: 3.20 3.3 3.40 V

5V ≤ VIN ≤ 16V, 10mA ≤ I

≤ 1A 3.14 3.46 V

LOAD

Undervoltage Lockout upper threshold 4.25 4.35 V

lower threshold 3.90 4.15 V
Feedback Bias Current MIC2179 [adj.] 60 150 nA

MIC2179-5.0, MIC2179-3.3 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 3.4 4.3 5.5 A

skip mode 600 mA
Switch On-Resistance high-side switch, VIN = 12V 160 350 mΩ

low-side switch, VIN = 12V 140 350 mΩ
Output Switch Leakage VSW = 16.5V 1 10 µA

June 1998 3 MIC2179

MIC2179

Micrel

Symbol Parameter Condition Min Typ Max Units

Enable Threshold 0.8 1.6 2.2 V

I

EN

Enable Leakage VEN = 0V to 5.5V –1 0.01 1 µA
PWM Threshold 0.6 1.1 1.4 V

I

PWM

PWM Leakage V

= 0V to 5.5V –1 0.01 1 µA

PWM

PWRGD Threshold MIC2179 [adj.]: measured at FB pin 1.09 1.13 1.17 V

MIC2179-5.0: measured at FB pin 4.33 4.54 4.75 V

MIC2179-3.3: measured at FB pin 2.87 3.00 3.13 V
PWRGD Output Low I
PWRGD Off Leakage V

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

= 1.0mA 0.25 0.4 V

SINK

= 5.5V 0.01 1 µA

PWRGD

MIC2179 4 June 1998

MIC2179

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

300

350

24681012141618

ON-RESISTANCE (mΩ)

INPUT VOLTAGE (V)

60

65

70

75

80

85

90

95

10 100 600

EFFICIENCY (%)

OUTPUT CURRENT (mA)

Typical Characteristics

Micrel

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

MIC2179-5.0

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

Reference Voltage

vs. Temperature

MIC2179 [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

MIC2179-3.3

-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)

5.5

5.3

5.1

4.9

4.7

4.5

4.3

4.1

CURRENT LIMIT (A)

3.9

3.7

3.5

-60 -30 0 30 60 90 120 150

12

10

8

6

June 1998 5 MIC2179

4

2

SUPPLY CURRENT (mA)

0

24681012141618

Current Limit

vs. Temperature

TEMPERATURE (°C)

PWM-Mode

Supply Current

OUTPUT

SWITCHING

INPUT VOLTAGE (V)

High-Side Switch

On-Resistance

125°C

85°C
25°C

Skip- and PWM-Mode

Efficiency

5.4V

PWM

8.4V
Skip

8.4V

5.4V
Skip

PWM

0°C

Low-Side Switch

On-Resistance

125°C

0

24681012141618

INPUT VOLTAGE (V)

ON-RESISTANCE (mΩ)

400
350
300
250
200
150
100

50

85°C
25°C

0°C

MIC2179

Block Diagram

V

IN

4.5V to 16.5V

VIN

Micrel

100µF

1716

C

C

Enable

Shutdown

Skip Mode

PWM Mode

Stop

0.01µF

R

C

EN

15

BIAS

14

PWM

5

SYNC

13

COMP

8

MIC2179 [Adjustable]

3.3V

Regulator

internal

supply Voltage

PWM/

Skip-Mode

Select

200kHz

Oscillator

UVLO,

Thermal

Shutdown

Corrective
Ramp

Reset
Pulse

1.245

L

C

to P

GND

R1

R2

OUT

*

GND

Output Good

R1

R2

1

V

OUT

V

SW

3

OUT

Output
Control

Logic

I

SENSE

Amp.

110mΩ

P-channel

D

110mΩ

N-channel

I

LIMIT

Comp.

I

LIMIT

Thresh.
Voltage

Skip-Mode

R

Q

S

PWM

Comp.

Comp.

V

SGND

1.245V

REF

9101112

Power Good

Comp.

1.13V

PGND

1
2
19
20

Bold lines indicate
high current traces

FB

7

V

IN

PWRGD

6

* Connect

S

20k

MIC2179 6 June 1998

MIC2179

Micrel

Functional Description

Micrel’s MIC2179 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. Its has internal
power MOSFETs that supply up to 1.5A load current and
operates with up to 100% duty cycle to allow low-dropout
operation. To optimize efficiency, the MIC2179 operates in
PWM and skip mode. Skip mode provides the best efficiency
when load current is less than 150mA, while PWM mode is
more efficient at higher current. PWM or skip-mode operation
is selected externally, allowing an intelligent system (i.e.
microprocessor controlled) to select the correct operating
mode for efficiency and noise requirements.

During PWM operation, the MIC2179 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 more detail.

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 150mΩ
when the MIC2179 operates from a 12V supply. Antishoot­through circuitry prevents the P-channel and N-channel from
turning on at the same time.

Current Limit

The MIC2179 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 4.3A.
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 to 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 MIC2179
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 MIC2179 has a low-current shutdown mode that is
controlled by the enable input (EN). When a logic 0 is applied
to EN, the MIC2179 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 MIC2179
control circuits. This internal supply is brought out to the BIAS
pin for bypassing by an external 0.01µF capacitor. Do not

connect an external load to the BIAS pin. It is not designed to
provide an external supply voltage.

Frequency Synchronization

The MIC2179 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 to 300kHz, and can operate with a minimum
pulse width of 500ns. If synchronization is not required,
connect SYNC to ground.

Power Good Flag

The power good flag (PWRGD) is an error flag that alerts a
system when the output is not in regulation. When the output
voltage is 10% below its nominal value, PWRGD is logic low,
signaling that V

is to low. PWRGD is an open-drain output

OUT

that can sink 1mA from a pull-up resistor connected to VIN.

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 MIC2179 operating in PWM mode
and 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 MIC2179 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

June 1998 7 MIC2179

MIC2179
(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 the average inductor
current . If V

is higher than nominal, the error amplifier

OUT

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 MIC2179 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 MIC2179 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 400mA. When IL1 reaches this value, the current limit
comparator sets the RS latch output to logic 0, turning off the

Micrel

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, and this
begins another switching cycle.

The skip-mode comparator regulates V
when the MIC2179 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 a

OUT

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

, or L1. This

OUT

allows the MIC2179 to always supply up to 300mA of load
current when operating in skip mode.

Selecting PWM- or Skip-Mode Operation

PWM or skip mode operation is selected by an external logic
signal applied to the PWM pin. A logic low places the
MIC2179 into PWM mode, and logic high places it into skip
mode. Skip mode operation provides the best efficiency
when load current is less than 150mA, and PWM operation is
more efficient at higher currents.

The MIC2179 was designed to be used in intelligent systems
that determine when it should operate in PWM or skip mode.
This makes the MIC2179 ideal for applications where a
regulator must guarantee low noise operation when supply­ing light load currents, such as cellular telephone, audio, and
multimedia circuits.

There are two important items to be aware of when selecting
PWM or skip mode. First, the MIC2179 can start-up only in
PWM mode, and therefore requires a logic low at PWM during
start-up. Second, in skip mode, the MIC2179 will supply a
maximum load current of approximately 300mA, so the
output will drop out of regulation when load current exceeds
this limit. To prevent this from occurring, the MIC2179 should
change from skip to PWM mode when load current exceeds
200mA.

MIC2179 8 June 1998

MIC2179

PWM-Mode Functional Diagram

V

IN

4.5V to 16.5V

VIN

Micrel

C

IN

1716

Stop

1.245

L1

I

L1

C

OUT

R1

R2

R1

1

V

OUT

SYNC

V

SW

3

OUT

I

SENSE

Amp.

110mΩ

P-channel

D

110mΩ

N-channel

Corrective

200kHz

Oscillator

13

Ramp

Reset
Pulse

PGND

1
2
19
20

FB

7

R2

R

Q

S

PWM

Comp.

Error
Amp.

COMP

8

R

C

C

C

V

1.245V

REF

MIC2179 [Adjustable] PWM-Mode Signal Path

9101112

SGND

V

SW

Reset
Pulse

I

L1

SENSE

LOAD

∆I

L1

I

Error Amp.
Output

I

June 1998 9 MIC2179

MIC2179

Skip-Mode Functional Diagram

Output Control Logic

One
Shot

I

LIMIT

Comp.

I

LIMIT

Thresh.
Voltage

Micrel

V

IN

4.5V to 16.5V

C

IN

VIN

1716

S

Q

R

I

SENSE

110mΩ

P-channel

Amp.

SW

3

V

OUT

D

PGND

1
2
19
20

1.245

L1

I

L1

C

OUT

R1

R2

1

V

OUT

MIC2179 [Adjustable] Skip-Mode Signal Path

V

IN

V

OUT

V

SW

0

One-Shot
Pulse

I

LIM

I

L1

0

Skip-Mode
Comp.

V

REF

SGND

R1

FB

7

R2

1.245V

9101112

V

+ 5mV

REF

V

FB

V

– 5mV

REF

MIC2179 10 June 1998

MIC2179

Micrel

Application Information

Feedback Resistor Selection (Adjustable Version)

The output voltage is programmed by connecting an external
resistive divider to the FB pin as shown in “MIC2179 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 with 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 of
thumb, 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)

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 0.2in, 5mm). Also, place a 0.1µF ceramic bypass
capacitor as close as possible to VIN.

Inductor Selection

The MIC2179 is a current-mode controller with internal slope
compensation. As a result, the inductor must be at least a
minimum value to prevent subharmonic oscillations. This
minimum value is calculated by the following formula:

L = V 3.0 H/V

MIN

OUT

×µ

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

L(peak)

LOAD(max)

Where:

∆I = V 1

L(max)

OUT





–1









The RMS current rating must

IN.

2

should be

MIN

I

∆

L(max)

+

2






−

V

V

IN(max)

OUT



×




5s

µ

L

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. Therefore, ESR

RIPPLE

must be equal or less than a maximum value calculated for a
specified V
age) and ∆I

ESR =

MAX

(typically less than 1% of the output volt-

RIPPLE

:

L(max)

V

RIPPLE

∆

I

L(max)

Typically, capacitors in the range of 100 to 220µF have ESR
less than this maximum value. The output capacitor can be
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
must be 1.4 × V

, and the voltage rating of an electrolytic

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 the other turns on. In skip mode, the
inductor current flows through the diode during the entire P­channel 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, the pole-zero reduces
loop-gain to a level referred to as the mid-band gain. The mid­band gain is low enough so that the loop gain crosses 0db
with sufficient phase margin. Typical values for the RC load
are 4.7nF to 10nF for the capacitor and 5kΩ to 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
MIC2179. 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.

June 1998 11 MIC2179

MIC2179

Micrel

The feedback resistors, RC compensation network, and
BIAS pin bypass capacitor should be located close to their
respective pins. To prevent ground bounce, their ground

currents returning to the power supply ground bus. SGND
and PGND should be tied together by a ground plane that
extends under the MIC2179.

traces and SGND should not be in the path of switching

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]

MIC2179 12 June 1998

MIC2179

Package Information

Micrel

0.875 (0.034)
REF

7.33 (0.289)

7.07 (0.278)

0.38 (0.015)

0.25 (0.010)

0.65 (0.0260)
BSC

5.40 (0.213)

5.20 (0.205)

7.90 (0.311)

7.65 (0.301)

2.00 (0.079)

1.73 (0.068)

0.21 (0.008)

0.05 (0.002)

COPLANARITY:

0.10 (0.004) MAX

20-lead SSOP (SM)

0°

–8°

10°

4°

DIMENSIONS:

MM (INCH)

0.22 (0.009)

0.13 (0.005)

1.25 (0.049) REF

0.95 (0.037)

0.55 (0.022)

June 1998 13 MIC2179

MIC2179

Micrel

MIC2179 14 June 1998

MIC2179

Micrel

June 1998 15 MIC2179

MIC2179

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.

© 1998 Micrel Incorporated

MIC2179 16 June 1998