MIC2141-BM5 MICREL [Micrel Semiconductor], MIC2141-BM5 Datasheet - Page 6

• Manufacturer Part Number: MIC2141-BM5
• Description: Micropower Boost Converter Preliminary Information
• Manufacturer: MICREL [Micrel Semiconductor]

MIC2141
Functional Diagram
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 A 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.
MIC2141
VC
FB
MIC2141
Reference
Bandgap
C
, the control input
FIXED DUTY CYCLE
Oscillator
330kHz
C
IN
. The
6
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.
C
) and the output ripple will be 6 times
GND
SW
C
) be held to 200mV less than
June 2000
Micrel
IN
of