MIC2142_07 Datasheet, PDF (6/17 Pages) Micrel Semiconductor – Micropower Boost Converter

Micrel, Inc.
Functional Diagram
VCC
Bandgap 1.265V
Reference
Oscillator
EN
Shutdown
330kHz
FIXED DUTY CYCLE
FB
MIC2142
MIC2142
SW
GND
Functional Description
This MIC2142 is a fixed duty cycle, constant frequency,
gated oscillator, micropower, switch-mode power supply
controller. Quiescent current for the MIC2142 is only
85µA in the switch off state, and since a MOSFET output
switch is used, additional switch drive current is
minimized. Efficiencies above 85% throughout most
operating conditions can be realized.
A functional block diagram is shown above and typical
schematic is shown on page 1. Regulation is performed
by a hysteretic comparator, which regulates the output
voltage by gating the internal oscillator. The internal
oscillator operates at a fixed 57% duty cycle and 330kHz
frequency. For the fixed output versions, the output is
divided down internally and then compared to the
internal V_REF input. An external resistive divider is use for
the adjustable version.
The 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 18mV, the
high frequency oscillator drive is removed from the
output switch. As the feedback input to the comparator
returns to the reference voltage level, the comparator is
reset and the high frequency oscillator is again gated to
the output switch. The 18mV of hysteresis seen at the
comparator will be multiplied by the ratio of the output
voltage to the reference voltage. For a five volt output
this ratio would be 4, corresponding to a ripple voltage of
72mV at the output.
The maximum output voltage is limited by the voltage
capability of the output switch. Output voltages up to 22V
can be achieved with a standard boost circuit. Higher
output voltages can be realized with a flyback
configuration.
October 2007
6
M9999-102507