March 1999

MIC5156/5157/5158

Micrel

MIC5156/5157/5158

Super LDOTM Regulator Controller

Final Information

either 3.3V, 5.0V, or 12V. The MIC5158 can be configured as
a fixed 5V controller or programmed to any voltage from 1.3V
to 36V using two external resistors.

The MIC5156 is available in an 8-pin DIP or SOP. The
MIC5157 and MIC5158 are available in a 14-pin DIP or SOP
which operate from 40

C to +85

Features

· 4.5mA typical operating current
· <1

A typical standby current

· Low external parts count
· Optional current limit (35mV typical threshold)
· 1% initial output voltage tolerance in most configurations
· 2% output voltage tolerance over temperature
· Fixed output voltages of 3.3V, 5.0V (MIC5156)
· Fixed output voltages of 3.3V, 5.0V, 12V (MIC5157)
· Programmable (1.3 to 36V) with 2 resistors (MIC5156/8)
· Internal charge pump voltage tripler (MIC5157/8)
· Enable pin to activate or shutdown the regulator
· Internal gate-to-source protective clamp
· All versions available in DIP and SOP

Applications

· Ultrahigh current ultralow dropout voltage regulator
· Constant high-current source
· Low parts count 5.0V to 3.3V computer supply
· Low noise/low-dropout SMPS post regulator
· High-current, current-limited switch

10A 5V to 3.3V Desktop Computer Regulator

General Description

The MIC5156, MIC5157, and MIC5158 Super Low-Dropout
(LDO) Regulator Controllers are single IC solutions for high-
current low-dropout linear voltage regulation. Super LDOTM
Regulators have the advantages of an external N-channel
power MOSFET as the linear pass element.

The MIC5156/7/8 family features a dropout voltage as low as
the R

DS(ON)

of the external power MOSFET multiplied by the

output current. The output current can be as high as the
largest MOSFETs can provide.

The MIC5156/7/8 family operates from 3V to 36V. The
MIC5156 requires an external gate drive supply to provide the
higher voltage needed to drive the gate of the external
MOSFET. The MIC5157 and MIC5158 each have an internal
charge pump tripler to produce the gate drive voltage. The
tripler is capable of providing enough voltage to drive a logic-
level MOSFET to 3.3V output from a 3.5V supply and is
clamped to 17.5V above the supply voltage. The tripler
requires three external capacitors.

The regulator output is constant-current limited when the
controller detects 35mV across an optional external sense
resistor. An active-low open-collector flag indicates a low
voltage of 8% or more below nominal output. A shutdown
(low) signal to the TTL-compatible enable control reduces
controller supply current to less than 1

A while forcing the

output voltage to ground.

The MIC5156-3.3 and MIC5156-5.0 controllers have inter-
nally fixed output voltages. The MIC5156 [adjustable] output
is configured using two external resistors. The MIC5157 is a
fixed output controller which is externally configured to select

MIC5157

C2+

GND

FLAG

3.3V

C1+

OUT

3.3V, 10A

(3.61V min.)

0.1µF

1.0µF

0.1µF

= 0.035V / I

LIMIT

IRLZ44 (Logic Level MOSFET)

47µF

* Improves transient

response to load changes

Enable

Shutdown

47µF

10A Low-Dropout Voltage Regulator

MIC5156-3.3

GND

FLAG

OUT

3.3V, 10A

0.1µF

= 0.035V / I

LIMIT

SMP60N03-10L

47µF

* Improves transient

response to load changes

+12V

Enable

Shutdown

47µF

Typical Applications

Super LDO is a trademark of Micrel, Inc.

Micrel, Inc. · 1849 Fortune Drive · San Jose, CA 95131 · USA · tel + 1 (408) 944-0800 · fax + 1 (408) 944-0970 · http://www.micrel.com

MIC5156/5157/5158

Micrel

MIC5156/5157/5158

March 1999

Pin Configuration

Ordering Information MIC5156

Part Number

Temperature Range

Voltage

Package

MIC5156-3.3BN

C to +85

3.3V

8-pin DIP

MIC5156-5.0BN

C to +85

5.0V

8-pin DIP

MIC5156BN

C to +85

Adjustable

8-pin DIP

MIC5156-3.3BM

C to +85

3.3V

8-pin SOP

MIC5156-5.0BM

C to +85

5.0V

8-pin SOP

MIC5156BM

C to +85

Adjustable

8-pin SOP

Ordering Information MIC5157

Part Number

Temperature Range

Voltage

Package

MIC5157BN

C to +85

Selectable

14-pin DIP

MIC5157BM

C to +85

Selectable

14-pin SOP

Ordering Information MIC5158

Part Number

Temperature Range

Voltage

Package

MIC5158BN

C to +85

5.0V/Adj.

14-pin DIP

MIC5158BM

C to +85

5.0V/Adj.

14-pin SOP

FLAG

S (Source)

GND

D (Drain)

D D

G (Gate)

MIC5156-x.x

5V FB

FLAG

GND

S (Source)

G (Gate)

D (Drain)

MIC5158

C2+

D D

C1+

3.3V

FLAG

GND

S (Source)

G (Gate)

D (Drain)

MIC5157

C2+

D D

C1+

FLAG

GND

D (Drain)

D D

G (Gate)

MIC5156

March 1999

MIC5156/5157/5158

Micrel

Pin Description MIC5156

Pin Number

Pin Name

Pin Function

Enable (Input): TTL high enables regulator; TTL low shuts down regulator.

FLAG

Output Flag (Output): Open collector output is active (low) when V

OUT

is more

than 8% below nominal output. Circuit has 3% hysteresis.

GND

Circuit ground.

N-channel Gate Drive Supply Voltage: User supplied voltage for driving the
gate of the external MOSFET.

Supply Voltage (Input): Supply voltage connection. Connect sense resistor
(R

) to V

if current limiting used. Connect supply bypass capacitor to

ground near device.

Gate (Output): Drives the gate of the external MOSFET.

Drain and Current Limit (Input): Connect to external MOSFET drain and
external sense resistor (current limit), or connect to V

and external MOSFET

drain (no current limit).

8 (3.3V, 5V)

Source (Input): Top of internal resistive divider chain. Connect directly to the
load for best load regulation.

8 (adjustable)

Error Amplifier (Input): Connect to external resistive divider.

Pin Description MIC5157, MIC5158

Pin Number

Pin Name

Pin Function

1 (MIC5157)

5V Configuration (Input): Connect to S (source) pin for 5V output.

1 (MIC5158)

Error Amplifier (Input): Connect to external resistive divider to obtain adjust-
able output.

2 (MIC5157)

3.3V

3.3V Configuration (Input): Connect to S (source) pin for 3.3V output.

2 (MIC5158)

5V FB

5V Feedback (Input): Connect to EA for fixed 5V output.

FLAG

Output Voltage Flag (Output): Open collector is active (low) when V

OUT

is 8%

or more below its nominal value.

GND

Circuit ground.

Voltage Tripler Output [Filter Capacitor]. Connect a 1 to 10

F capacitor to ground.

Charge Pump Capacitor 2: Second stage of internal voltage tripler. Connect a
0.1

F capacitor from C2+ to C2.

C2+

Charge Pump Capacitor 2: See C2 pin 6.

C1+

Charge Pump Capacitor 1: First stage of internal voltage tripler. Connect a
0.1

F capacitor from C1+ to C1.

Charge Pump Capacitor 1: See C1+ pin 8.

Supply Voltage (Input): Supply voltage connection. Connect sense resistor
(R

) to V

if current limiting used. Connect supply bypass capacitor to

ground near device.

Gate (Output): Connect to External MOSFET gate.

Drain and Current Limit (Input): Connect to external MOSFET drain and
external sense resistor (current limit), or connect to V

and external MOSFET

drain (no current limit).

13 (MIC5157)

Source and 3.3V/5V Configuration: Top of internal resistor chain. Connect to
source of external MOSFET for 3.3V, 5V, and 12V operation. Also see 3.3V
and 5V pin descriptions.

13 (MIC5158)

Source (Input): Top of internal resistor chain. Connect to top of external
resistive divider and source of external MOSFET.

Enable (Input): TTL high enables regulator; TTL low shuts down regulator.

MIC5156/5157/5158

Micrel

MIC5156/5157/5158

March 1999

Electrical Characteristics

(Note 5)

= 5V, V

= 5V; T

= 25

C; unless noted.

Symbol

Parameter

Condition

Min

Typ

Max

Units

Supply Voltage

DD(ON)

Supply Current MIC5156

Operating, V

= 5V

2.7

DD(OFF)

Shutdown, V

= 0V

0.1

DD(ON)

Supply Current MIC5157/8

Operating, V

= 5V

4.5

DD(OFF)

Shutdown, V

= 0V

0.1

Enable Input Threshold

High

2.4

1.3

Low

1.3

0.8

EN I

Enable Input Bias Current

= 2.4V

Max. Charge Pump Voltage

, V

> 10V

17.5

18.5

Charge Pump Frequency

160

kHz

OUT MAX

Maximum Gate Drive Voltage

SOURCE

= 0V

(MIC5157/8)

= 3.5V

7.0

= 5V

11.3

= 12V

OUT MIN

Minimum Gate Drive Voltage

SOURCE

> V

OUT(NOM)

1.0

LIM

Current Limit Threshold

@ I

LIM

Source Voltage

Short G (gate) to (S) source, Note 4
MIC5156-3.3

3.267

3.3

3.333

MIC5156-5.0

4.950

5.0

5.050

MIC5157, 3.3V pin to S pin (3.3V config.)

3.250

3.3

3.350

MIC5157, 5V pin to S pin (5V config.)

4.950

5.0

5.050

MIC5157, V

= 7V, (12V config.)

11.70

12.30

MIC5158, 5V FB pin to EA pin (5V config.)

4.925

5.0

5.075

Bandgap Reference Voltage

MIC5156 [adjustable] and MIC5158

1.222

1.235

1.248

Output Voltage Line Regulation

5V < V

< 15V, V

OUT

= 3.3V

GS MAX

Gate to Source Clamp

16.6

Flag Comparator Threshold

% of nominal V

SOURCE

Flag Comparator Hysteresis

% of nominal V

SOURCE

SAT

Flag Comparator Sat. Voltage

FLAG

= 1mA

0.09

0.2

Note 1.

Exceeding the absolute maximum rating may damage the device.

Note 2.

The device is not guaranteed to function outside its operating rating.

Note 3.

Devices are ESD sensitive. Handling precautions recommended.

Note 4.

Test configuration. External MOSFET not used.

Note 5.

Specification for packaged product only.

Absolute Maximum Ratings

(Note 1)

Supply Input (V

) ...................................................... +38V

Enable Input (V

) ......................................... 0.3V to 36V

Gate Output (V

) MIC5156 ......................................... +55V

Charge Pump Node (V

) MIC5157/8 ........................ +55V

Source Connection (V

) .................................... 1.3 to +36V

Flag (V

FLAG

) .................................................... 0.3 to +40V

Storage Temperature (T

) ....................... 65

C to +150

Lead Temperature (soldering 10 sec.) ...................... 300

Operating Ratings

(Note 2)

Ambient Temperature Range (T

)

MIC515xBM/BN ..................................... 40

C to +85

Junction Temperature (T

) ...................................... +150

Thermal Resistance (

)

Package

MIC5156

MIC5157/8

DIP ............................... 100

C/W ..................... 90

C/W

SOP .............................. 160

C/W ................... 120

C/W

March 1999

MIC5156/5157/5158

Micrel

Typical Characteristics

3.26

3.27

3.28

3.29

3.30

3.31

3.32

3.33

3.34

-60 -30

90 120 150

OUTPUT VOLTAGE (V)

TEMPERATURE (

3.3V Regulator Output

Voltage vs. Temperature

4.96

4.97

4.98

4.99

5.00

5.01

5.02

5.03

5.04

-60 -30

90 120 150

OUTPUT VOLTAGE (V)

TEMPERATURE (

5.0V Regulator Output

Voltage vs. Temperature

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

ON-STATE SUPPLY CURRENT (mA)

SUPPLY VOLTAGE (V)

MIC5157/8 On-State Supply

Current vs. Supply Voltage

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-60 -30

90 120 150

ON-STATE SUPPLY CURRENT (mA)

TEMPERATURE (

MIC5157/8 On-State Supply

Current vs. Temperature

= 5V

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-60 -30

90 120 150

OFF-STATE SUPPLY CURRENT (

TEMPERATURE (

Off-State Supply Current

vs. Temperature

= 5V

CHARGE PUMP VOLTAGE (V)

SUPPLY VOLTAGE (V)

Charge-Pump Output Voltage

vs. Supply Voltage

100

125

150

175

200

-60 -30

90 120 150

FLAG VOLTAGE (mV)

TEMPERATURE (

Flag Output Voltage

vs. Temperature

= 5V

FLAG

= 1mA

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

-60 -30

90 120 150

ENABLE THRESHOLD VOLTAGE (V)

TEMPERATURE (

Enable Threshold Voltage

vs. Temperature

-60 -30

90 120 150

CURRENT LIMIT THRESHOLD (mV)

TEMPERATURE (

Current Limit Threshold

vs. Temperature

-2

-0.2

0.0

0.2

0.4

0.6

VOLTAGE (V)

TIME (ms)

MIC5157/8 Turn-On

Response Time for 3.3V

= 1

= 50

LOGIC

INPUT

3.3V

OUTPUT

MOSFET = IRF540
V

= 5V, I

= 0.5A

= C

= 0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FLAG VOLTAGE (V)

FLAG SINK CURRENT (mA)

Flag Output Voltage

vs. Flag Current

= 5V

100

120

10 12 14 16

ENABLE BIAS CURRENT (

ENABLE VOLTAGE (V)

Enable Input Bias Current

vs. Enable Voltage

MIC5156/5157/5158

Micrel

MIC5156/5157/5158

March 1999

Block Diagram MIC5156

Enable
Shutdown

1.235V

Bandgap

Reference

Internal
Bias

Error

Amp

75mV

35mV

GND

S* (Source)

16.6V

LIMIT

Comparator

D (Drain)

LIMIT

]

G (Gate)

FLAG

OUT

Comparator

10k

to all

internal blocks

17k

Load 1

Regulated
+3.3V Output

Load 2

+12V Input

+5V Input

0.1µF

Q1
SMP60N03-10L

Switched

5V Load

12k

* fixed version only

3.3V = 17k, 5V = 32k

adjustable version only

Block Diagram MIC5157

Block Diagram with External Components

Fixed 3.3V Power Supply with 5.0V Load Switch

Block Diagram with External Components

Fixed 3.3V 10A Power Supply

1.235V

Bandgap

Reference

Clamp

Internal
Bias

Error

Amp

75mV

35mV

GND

S (Source)*

LIMIT

Comparator

D (Drain)

LIMIT

]

G (Gate)

OUT

Comparator

to all

internal blocks

3.3V

Load

Regulated
+3.3V Output

+5V Input

Q1
IRFZ44

1µF

16.6V

10k

58k

15k

17k

Oscillator

Charge Pump

Tripler

C1+

C2+

0.1µF

FLAG

Enable
Shutdown

March 1999

MIC5156/5157/5158

Micrel

Functional Description

Super LDO Regulator is a complete regulator built around

Micrel's

Super LDO Regulator Controller.

Refer to Block Diagrams MIC5156, MIC5157, and MIC5158.

Version Differences

The MIC5156 requires an external voltage for MOSFET gate
drive and is available in 3.3V fixed output, 5V fixed output, or
adjustable output versions. With 8-pins, the MIC5156 is the
smallest of the Super LDO Regulator Controllers.

The MIC5157 and MIC5158 each have an internal charge
pump which provides MOSFET gate drive voltage. The
MIC5157 has a selectable fixed output of 3.3V, 5V, or 12V.
The MIC5158 may be configured for a fixed 5V or adjustable
output.

Enable (EN)

With at least 3.0V on V

, applying a TTL low to EN places

the controller in shutdown mode. A TTL high on EN enables
the internal bias circuit which powers all internal circuitry. EN
must be pulled high if unused. The voltage applied to EN may
be as high as 36V.

The controller draws less than 1

A in shutdown mode.

Gate Enhancement

The Super LDO Regulator Controller manages the gate-to-
source enhancement voltage for an external N-channel

1.235V

Bandgap

Reference

Clamp

Internal
Bias

Error

Amp

75mV

35mV

S (Source)

LIMIT

Comparator

D (Drain)

LIMIT

]

G (Gate)

OUT

Comparator

to all

internal blocks

GND

Regulated
+3.6V Output

+5V Input

Q1
IRFZ44

1µF

16.6V

32k

10k

5V
FB

Oscillator

Charge Pump

Tripler

C1+

C2+

0.1µF

FLAG

Load

19.1k

10.0k

Enable
Shutdown

Block Diagram MIC5158

Block Diagram with External Components

Adjustable Power Supply, 3.6V Configuration

MOSFET (regulator pass element) placed between the sup-
ply and the load. The gate-to-source voltage may vary from
1V to 16V depending upon the supply and load conditions.

Because the source voltage (output) approaches the drain
voltage (input) when the regulator is in dropout and the
MOSFET is fully enhanced, an additional higher supply
voltage is required to produce the necessary gate-to-source
enhancement. This higher gate drive voltage is provided by
an external gate drive supply (MIC5156) or by an internal
charge pump (MIC5157 and MIC5158).

Gate Drive Supply Voltage (MIC5156 only)

The gate drive supply voltage must not be more than 14V
above the supply voltage (V

< 14V). The minimum

necessary gate drive supply voltage is:

= V

OUT

+ V

+ 1

where:

= gate drive supply voltage

OUT

= regulator output voltage

= gate-to-source voltage for full

MOSFET gate enhancement

The error amplifier uses the gate drive supply voltage to drive
the gate of the external MOSFET. The error amplifier output
can swing to within 1V of V

MIC5156/5157/5158

Micrel

MIC5156/5157/5158

March 1999

Charge Pump (MIC5157/5158 only)

The charge pump tripler creates a dc voltage across reservoir
capacitor C3. External capacitors C1 and C2 provide the
necessary storage for the stages of the charge pump tripler.

The tripler's approximate dc output voltage is:

3 (V

where:

= charge pump output voltage

= supply voltage

The V

clamp circuit limits the charge pump voltage to 16V

above V

by gating the charge pump oscillator ON or OFF

as required. The charge pump oscillator operates at 160kHz.

The error amplifier uses the charge pump voltage to drive the
gate of the external MOSFET. It provides a constant load of
about 1mA to the charge pump. The error amplifier output can
swing to within 1V of V

Although the MIC5157/8 is designed to provide gate drive
using its internal charge pump, an external gate drive supply
voltage can be applied to V

. When using an external gate

drive supply, V

must not be forced more than 14V higher

than V

When constant loads are driven, the ON/OFF switching of the
charge pump may be evident on the output waveform. This is
caused by the charge pump switching ON and rapidly in-
creasing the supply voltage to the error amplifier. The period
of this small charge pump excitation is determined by a
number of factors: the input voltage, the 1mA op-amp load,
any dc leakage associated with the MOSFET gate circuit, the
size of the charge pump capacitors, the size of the charge
pump reservoir capacitor, and the characteristics of the input
voltage and load. The period is lengthened by increasing the
charge pump reservoir capacitor (C3). The amplitude is
reduced by weakening the charge pump--this is accom-
plished by reducing the size of the pump capacitors (C1 and
C2). If this small burst is a problem in the application, use a
10

F reservoir capacitor at C3 and 0.01

F pump capacitors

at C1 and C2. Note that the recovery time to repetitive load
transients may be affected with small pump capacitors.

Gate-to-Source Clamp

A gate-to-source protective voltage clamp of 16.6V protects
the MOSFET in the event that the output voltage is suddenly
forced to zero volts. This prevents damage to the external
MOSFET during shorted load conditions. Refer to "Charge
Pump" for normal clamp circuit operation.

The source connection required by the gate-to-source clamp
is not available on the adjustable version of the MIC5156.

Output Regulation

At start-up, the error amplifier feedback voltage (EA), or
internal feedback on fixed versions, is below nominal when
compared to the internal 1.235V bandgap reference. This
forces the error amplifier output high which turns on external
MOSFET Q1. Once the output reaches regulation, the con-
troller maintains constant output voltage under changing
input and load conditions by adjusting the error amplifier
output voltage (gate enhancement voltage) according to the
feedback voltage.

Out-of-Regulation Detection

When the output voltage is 8% or more below nominal, the
open-collector FLAG output (normally high) is forced low to
signal a fault condition. The FLAG output can be used to
signal or control external circuitry. The FLAG output can also
be used to shut down the regulator using the EN control.

Current Limiting

Super LDO Regulators perform constant-current limiting (not
foldback). To implement current limiting, a sense resistor
(R

) must be placed in the "power" path between V

and D

(drain).

If the voltage drop across the sense resistor reaches 35mV,
the current limit comparator reduces the error amplifier out-
put. The error amplifier output is decreased only enough to
reduce the output current, keeping the voltage across the
sense resistor from exceeding 35mV.

Application Information

MOSFET Selection

Standard N-channel enhancement-mode MOSFETs are ac-
ceptable for most Super LDO regulator applications.

Logic-level N-channel enhancement-mode MOSFETs may
be necessary if the external gate drive voltage is too low
(MIC5156), or the input voltage is too low, to provide ad-
equate charge pump voltage (MIC5157/8) to enhance a
standard MOSFET.

Circuit Layout

For the best voltage regulation, place the source, ground, and
error amplifier connections as close as possible to the load.
See figures (1a) and (1b).

GND

Load

MIC515x

Figure 1a. Connections for Fixed Output

March 1999

MIC5156/5157/5158

Micrel

Adjustable Configurations

Micrel's MIC5156 [adjustable] and MIC5158 require an exter-
nal resistive divider to set the output voltage from 1.235V to
36V. For best results, use a 10k

resistor for R2. See

equation (1) and figure (2).

= 1

OUT

1.235

GND

EA*

MIC5157/8

R2
10k

OUT

Figure 2. Typical Resistive Divider

Input Filter Capacitor

The Super LDO requires an input bypass capacitor for
accommodating wide changes in load current and for decou-
pling the error amplifier and charge pump. A medium to large
value low-ESR (equivalent series resistance) capacitor is
best, mounted close to the device.

Output Filter Capacitor

An output filter capacitor may be used to reduce ripple and
improve load regulation. Stable operation does not require a
large capacitor, but for transient load regulation the size of the
output capacitor may become a consideration. Common
aluminum electrolytic capacitors perform nicely; very low-
ESR capacitors are not necessary. Increased capacitance
(rather than reduced ESR) is preferred. The capacitor value
should be large enough to provide sufficient I = C

dV/dt

current consistent with the required transient load regulation
quality. For a given step increase in load current, the output
voltage will drop by about dV = I

dt/C, where I represents the

increase in load current over time t. This relationship as-
sumes that all output current was being supplied via the
MOSFET pass device prior to the load increase. Small
(0.01

F to 10

F) film capacitors parallel to the load will further

improve response to transient loads.

Some linear regulators specify a minimum required output
filter capacitance because the capacitor determines the
dominant pole of the system, and thereby stabilizes the
system. This is not the situation for the MIC5156/7/8; its
dominant pole is determined within its error amplifier.

GND

Load

MIC5157

MIC5158

Figure 1b. Connections for Adjustable Output

GND

Load

MIC5156

* Optional 16V zener diode

recommended in applications
where V

is greater than 18V

Figure 1c. MIC5156 Connections for

Adjustable Output

MOSFET Gate-to-Source Protection

When using the adjustable version of the MIC5156, an
external 16V zener diode placed from gate-to-source is
recommended for MOSFET protection. All other versions of
the Super LDO regulator controller use the internal gate-to-
source clamp.

Output Voltage Configuration

Fixed Configurations

The MIC5156-3.3 and MIC5156-5.0 are preset for 3.3V and
5.0V respectively.

The MIC5157 operates at 3.3V when the 3.3V pin is con-
nected to the S (source) pin; 5.0V when the 5.0V pin is
connected to the S pin; or 12V if the 3.3V and 5.0V pins are
open.

The MIC5158 operates at a fixed 5V (without an external
resistive divider) if the 5V FB pin is connected to EA.

MIC5156/5157/5158

Micrel

MIC5156/5157/5158

March 1999

Current Limiting

Current sensing requires a low-value series resistance (R

)

between V

and D (drain). Refer to the typical applications.

The internal current-limiting circuit limits the voltage drop
across the sense resistor to 35mV. Equation (2) provides the
sense resistor value required for a given maximum current.

35mV

LIM

where:

= sense resistor value

LIM

= maximum output current

Most current-limited applications require low-value resistors.
See

Application Hints 21 and 25 for construction hints.

Non-Current-Limited Applications

For circuits not requiring current limiting, do not use a sense
resistor between V

and D (drain). See figure (3). The

controller will not limit current when it does not detect a 35mV
drop from V

to D.

MIC5156

Figure 3. No Current Limit

3.3V Microprocessor Applications

For computer designs that use 3.3V microprocessors with 5V
logic, the FLAG output can be used to suppress the 5V supply
until the 3.3V output is in regulation. Refer to the external
components shown with the MIC5156 Block Diagram.

SMPS Post Regulator Application

A Super LDO regulator can be used as a post regulator for a
switch-mode power supply. The Super LDO regulator can
provide a significant reduction in peak-to-peak ripple voltage.

High-Current Switch Application

All versions of the MIC5156/7/8 may be used for current-
limited, high-current, high-side switching with or without
voltage regulation. See figure (4a). Simply leave the "S"
terminal open. A 16V zener diode from the gate to the source
of the MOSFET protects the MOSFET from overdrive during
fault conditions.

MIC5156-x.x

Gate Supply

Enable

Shutdown

GND

Load

Figure 4a. High-Side Switch

If a MIC5157 or MIC5158 is used and is shutdown for a given
time, the charge pump reservoir V

will bleed off. If recharg-

ing the reservoir causes an unacceptable delay in the load
reaching its operating voltage, do not use the EN pin for on/
off control. Instead, use the MIC5158, hold EN high to keep
the charge pump in continuous operation, and switch the
MOSFET on or off by overriding the error amplifier input as
shown in figure (4b).

GND

Load

MIC5158

Output Off
Output On

1N4148

Figure 4b. Fast High-Side Switch

Battery Charger Application

The MIC5158 may be used in constant-current applications
such as battery chargers. See figure (5). The regulator
supplies a constant-current (35mV

R3) until the battery

approaches the float voltage:

= 1. 235 1 +

where:

= float voltage

At float voltage, the MOSFET is shut off. A trickle charge is
supplied by R4.

March 1999

MIC5156/5157/5158

Micrel

GND

MIC5158

Figure 5. Battery Charger Concept

Uninterruptible Power Supply

The MIC5157 and two N-channel MOSFETs provide battery
switching for uninterruptible power as shown in figure (6).
Two MOSFETs are placed source-to-source to prevent cur-
rent flow through their body diodes when switched off. The
Super LDO regulator is continuously enabled to achieve fast
battery switch-in. Careful attention must be paid to the ac-line
monitoring circuitry to ensure that the output voltage does not
fall below design limits while the battery is being switched in.

Line

Battery

MIC5158

GND

1N4148

Uninterruptable
DC