MIC2141

Micrel

MIC2141

6

June 2000

Functional Diagram

330kHz

FIXED DUTY CYCLE

VC

FB

IN

GND

SW

Oscillator

MIC2141

Bandgap

Reference

Functional Description

See “Applications Information” for component selection and

predesigned circuits.

Overview

This MIC2141 is a fixed-duty-cycle, constant-frequency, gated-

oscillator, micropower, switch-mode power supply controller.

Quiescent current for the MIC2141 is only 70

µA in the switch

off state, and since a MOSFET output switch is used, addi-

tional current needed for switch drive is minimized. Efficien-

cies above 85% throughout most operating conditions can be

realized.

Regulaton

Regulation is performed by a hysteretic comparator which

regulates the output voltage by gating the internal oscillator.

The user applies a programming voltage to the VC pin. (For

a fixed or adjustable output regulator, with an internal refer-

ence, use the MIC2142.) The output voltage is divided down

internally and then compared to the V

voltage, forcing the output voltage to 6 times the V

comparator has hysteresis built into it, which determines the

amount of low frequency ripple that will be present on the

output. Once the feedback input to the comparator exceeds

the control voltage by 10mV, the high-frequency oscillator

drive is removed from the output switch. As the feedback

input to the comparator returns to the control voltage level,

the comparator is reset and the high-frequency oscillator is

again gated to the output switch. Typically 10mV of hysteresis

seen at the comparator will correspond to 60mV of low-

frequency ripple at the output. Applications, which require

continuous adjustment of the output voltage, can do so by

adjustment of the VC control pin.

Output

The maximum output voltage is limited by the voltage capa-

bility of the output switch. Output voltages up to 22V can be

achieved with a standard boost circuit. Higher output volt-

ages require a flyback configuration.

Output Voltage Control

The internal hysteretic comparator disables the output drive

once the output voltage exceeds the nominal by 30mV. The

drive is then enabled once the output voltage drops below the

nominal by 30mV.

The reference level, which actually programs the output

voltage, is set by the VC control input. The output is 6 times

the control voltage (V

the comparator hystersis. Therefore, with 10mV of hystersis,

there will be

±30mV variation in the output around the nominal

value. See the “Typical Characteristics: Control Voltage vs.

Output Voltage” for a graph of input-to-output behavior.

The common-mode range of the comparator requires that the

maximum control voltage (V

VIN. When programming for a 20V output, a minimum V

3.5V will be required. See the “Typical Characteristics: Gain

vs. Output Voltage” for a graph of gain behavior. To achieve

20V output at lower input voltages, the external resistive

divider (R1 and R2) shown in Figure 2 can be added. This

circuit will increase the control-to-output gain, while limiting

the error introduced by the tolerance of the internal resistor

feedback network.