CMOS Micropower Step-Up
Switching Regulator
Pin Description
PIN
NAME
FUNCTION
Low-Battery Detection Comparator Input. The LBD output, pin 8, sinks current whenever this pin is
below the low-battery detector threshold, typically 1.31V.
1
LBR
An external capacitor connected between this terminal and ground sets the oscillator frequency.
47pF = 40 kHz.
2
3
C
X
This pin drives the external inductor. The internal N-channel MOSFET that drives L has an output
X
resistance of 4Ω and a peak current rating of 525mA.
L
X
4
5
GND
+V
Ground
The positive supply voltage, from 2.0V to 16.5V (MAX630).
S
The MAX630/MAX4193 shut down when this pin is left floating or is driven below 0.2V. For normal
6
I
operation, connect I directly to +V or drive it high with either a CMOS gate or pullup resistor
C S
C
connected to +V . The supply current is typically 10nA in the shutdown mode
S
The output voltage is set by an external resistive divider connected from the converter output to V
FB
and ground. The MAX630/MAX4193 pulse the L output whenever the voltage at this terminal is less
X
than 1.31V.
7
8
V
FB
The Low-Battery Detector output is an open-drain N-channel MOSFET that sinks up to 600μA (typ)
whenever the LBR input, pin 1, is below 1.31V.
LBD
Output Driver (L Pin)
X
Detailed Description
The MAX630/MAX4193 output device is a large
N-channel MOSFET with an on-resistance of 4Ω and a
peak current rating of 525mA. One well-known advan-
tage that MOSFETs have over bipolar transistors in
switching applications is higher speed, which reduces
switching losses and allows the use of smaller, lighter,
less costly magnetic components. Also important is that
MOSFETs, unlike bipolar transistors, do not require
base current that, in low-power DC-DC converters,
often accounts for a major portion of input power.
The operation of the MAX630 can best be understood
by examining the voltage regulating loop of Figure 1.
R1 and R2 divide the output voltage, which is com-
pared with the 1.3V internal reference by comparator
COMP1. When the output voltage is lower than desired,
the comparator output goes high and the oscillator out-
put pulses are passed through the NOR gate latch,
turning on the output N-channel MOSFET at pin 3, L .
X
As long as the output voltage is less than the desired
voltage, pin 3 drives the inductor with a series of pulses
at the oscillator frequency.
The operating current of the MAX630 and MAX4193
increases by approximately 1µA/kHz at maximum
power output due to the charging current required by
the gate capacitance of the L output driver (e.g., 40µA
X
increase at a 40kHz operating frequency). In compari-
son, equivalent bipolar circuits typically drive their NPN
output device with 2mA of base drive, causing the
bipolar circuit’s operating current to increase by a fac-
tor of 10 between no load and full load.
Each time the output N-channel MOSFET is turned on,
the current through the external coil, L1, increases,
storing energy in the coil. Each time the output turns off,
the voltage across the coil reverses sign and the volt-
age at L rises until the catch diode, D1, is forward
X
biased, delivering power to the output.
L
X
When the output voltage reaches the desired level,
1.31V x (1 + R1 / R2), the comparator output goes low
and the inductor is no longer pulsed. Current is then
supplied by the filter capacitor, C1, until the output volt-
Oscillator
The oscillator frequency is set by a single external, low-
cost ceramic capacitor connected to pin 2, C . 47pF
X
age drops below the threshold, and once again L is
X
sets the oscillator to 40kHz, a reasonable compromise
between lower switching losses at low frequencies and
reduced inductor size at higher frequencies.
switched on, repeating the cycle. The average duty
cycle at L is directly proportional to the output current.
X
4
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