MIC5156/5157/5158

Micrel, Inc.

MIC5156/5157/5158

8

August 2005

Charge Pump (MIC5157/5158 only)

The charge pump tripler creates a dc voltage across reservoir

capacitor C3. External capacitors C1 and C2 provide the nec-

essary storage for the stages of the charge pump tripler.

The tripler’s approximate dc output voltage is:

where:

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

Although the MIC5157/8 is designed to provide gate drive

using its internal charge pump, an external gate drive sup-

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

increasing the supply voltage to the error amplifier.The period

ofthissmallchargepumpexcitationisdeterminedbyanumber

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

reservoircapacitor,andthecharacteristicsoftheinputvoltage

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 accomplished by reduc-

ing the size of the pump capacitors (C1 and C2). If this small

burst is a problem in the application, use a 10µF reservoir

capacitor at C3 and 0.01µF 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 exter-

nal MOSFET Q1. Once the output reaches regulation, the

controller 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

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 adequate

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

S

G

MIC515x

Figure 1a. Connections for Fixed Output