August 2005

9

MIC5156/5157/5158

MIC5156/5157/5158

Micrel, Inc.

Adjustable Configurations

Micrel’s MIC5156 [adjustable] and MIC5158 require an ex-

ternal resistive divider to set the output voltage from 1.235V

to 36V. For best results, use a 10kΩ resistor for R2. See

equation (1) and figure (2).

1)

(

- 1

)

1.235

GND

S

EA*

MIC5157/8

R1

R2

10k

G

Figure 2. Typical Resistive Divider

Input Filter Capacitor

TheSuperLDOrequiresaninputbypasscapacitorforaccom-

modating wide changes in load current and for decoupling

the error amplifier and charge pump.Amedium 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

aluminumelectrolyticcapacitorsperformnicely;verylow-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.

Foragivenstepincreaseinloadcurrent,theoutputvoltagewill

drop by about dV = I × dt/C, where I represents the increase

in load current over time t. This relationship assumes that

all output current was being supplied via the MOSFET pass

device prior to the load increase. Small (0.01µF to 10µF) film

capacitors parallel to the load will further improve response

to transient loads.

Somelinearregulatorsspecifyaminimumrequiredoutputfilter

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

S

EA

MIC5157

or

MIC5158

G

Figure 1b. Connections for Adjustable Output

GND

MIC5156

G

EA

* Optional 16V zener diode

recommended in applications

*

Figure 1c. MIC5156 Connections for

Adjustable Output

MOSFET Gate-to-Source Protection

WhenusingtheadjustableversionoftheMIC5156,anexternal

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.

TheMIC5157operatesat3.3Vwhenthe3.3Vpinisconnected

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 5VFB pin is connected to EA.