MIC5156/5157/5158

Micrel, Inc.

MIC5156/5157/5158

10

August 2005

Current Limiting

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.

2)

35mV

where:

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

troller will not limit current when it does not detect a 35mV

S

G

MIC5156

D

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 sup-

ply 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-modepowersupply.TheSuperLDOregulatorcanpro-

vide 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-lim-

ited, 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.

G

MIC5156-x.x

D

Gate Supply

Enable

Shutdown

EN

GND

S

Figure 4a. High-Side Switch

If a MIC5157 or MIC5158 is used and is shutdown for a

recharging 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

S

EA

MIC5158

G

EN

Output Off

Output On

1N4148

Figure 4b. Fast High-Side Switch

Battery Charger Application

The MIC5158 may be used in constant-current applications

suchasbatterychargers.Seefigure(5).Theregulatorsupplies

a constant-current (35mV ÷ R3) until the battery approaches

the float voltage:

)

R1

R2

where:

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

supplied by R4.