CMOS Micropower Step-Up
Switching Regulator
LOW BATTERY INPUT
+5V INPUT
R3
169kΩ
LBD 8
LOW-BATTERY OUTPUT
(LOW IF INPUT < 3V)
MAX630
1 LBR
R4
100kΩ
COMP 2
1.31V
L1
470
R2
47.5kΩ
2 C
X
OSC
V
7
FB
R1
499kΩ
COMP 1
40kHz
C
C
SHUTDOWN
OPERATE
3 L
X
I
6
5
C
R
≅ 3Ω
ON
D1
1N4148
1.31V
BANDGAP
REFERENCE
AND
+V
S
4 GND
BIAS GENERATOR
+15V OUTPUT
20mA
C1
470μF
25V
Figure 1. +5V to +15V Converter and Block Diagram
MAX630’s analog circuitry, oscillator, L , and LBD out-
X
puts are turned off. The device’s quiescent current dur-
ing shutdown is typically 10nA (1µA max).
Low-Battery Detector
The low-battery detector compares the voltage on LBR
with the internal 1.31V reference. The output, LBD, is an
open-drain N-channel MOSFET. In addition to detecting
and warning of a low battery voltage, the comparator
can also perform other voltage-monitoring operations
such as power-failure detection.
Bootstrapped Operation
In most circuits, the preferred source of +V voltage for
S
the MAX630 and MAX4193 is the boosted output volt-
age. This is often referred to as a “bootstrapped” oper-
ation since the circuit figuratively “lifts” itself up.
Another use of the low-battery detector is to lower the
oscillator frequency when the input voltage goes below
a specified level. Lowering the oscillator frequency
increases the available output power, compensating for
the decrease in available power caused by reduced
input voltage (see Figure 5).
The on-resistance of the N-channel L output decreas-
X
es with an increase in +V ; however, the device operat-
S
ing current goes up with +V (see the Typical
S
Operating Characteristics, I vs. +V graph). In circuits
S
S
with very low output current and input voltages greater
than 3V, it may be more efficient to connect +V direct-
S
Logic-Level Shutdown Input
ly to the input voltage rather than bootstrap.
The shutdown mode is entered whenever I (pin 6) is
C
driven below 0.2V or left floating. When shut down, the
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5
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
