MIC2141_06 Datasheet, PDF (6/16 Pages) Micrel Semiconductor – Micropower Boost Converter

Micrel, Inc.
Functional Diagram
IN
Bandgap
Reference
Oscillator
330kHz
FIXED DUTY CYCLE
VC
FB
MIC2141
SW
MIC2141
Functional Description
See “Applications Information” for component selection
and pre-designed 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
70µA in the switch off state, and since a MOSFET output
switch is used, additional current needed for switch drive
is minimized. Efficiencies above 85% throughout most
operating conditions can be realized.
Regulation
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
V_C pin. (For a fixed or adjustable output regulator, with
an internal reference, use the MIC2142.) The output
voltage is divided down internally and then compared to
the V_C, the control input voltage, forcing the output
voltage to 6 times the V_C. 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 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 V_C control pin.
GND
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 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 V_C control input. The output is 6
times the control voltage (V_C) and the output ripple will
be 6 times 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_C) be held to 200mV
less than V_IN. When programming for a 20V output, a
minimum V_IN of 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.
December 2006
6
M9999-122006