CMOS Micropower Step-Up
Switching Regulator
voltages that are either greater or less than the output.
DC-DC converters can also be classified by the control
method. The two most common are pulse-width modu-
lation (PWM) and pulse-frequency modulation (PFM).
PWM switch-mode power-supply ICs (of which current-
mode control is one variant) are well-established in
high-power off-line switchers. Both PWM and PFM cir-
cuits control the output voltage by varying duty cycle.
In the PWM circuit, the frequency is held constant and
the width of each pulse is varied. In the PFM circuit, the
pulse width is held constant and duty cycle is con-
trolled by changing the pulse repetition rate.
The MAX630 refines the basic PFM by employing a con-
stant-frequency oscillator. Its output MOSFET is switched
on when the oscillator is high and the output voltages is
lower than desired. If the output voltage is higher than
desired, the MOSFET output is disabled for that oscillator
cycle. This pulse skipping varies the average duty cycle,
and thereby controls the output voltage.
Note that, unlike the PWM ICs, which use an op amp as
the control element, the MAX630 uses a comparator to
compare the output voltage to an onboard reference.
This reduces the number of external components and
operating current.
MAX630/MAX4193
+5V
6
1MΩ
7
95.3kΩ
V
FB
5
I
C
+V
S
330μF
25V
MAX630
L
X
2
47pF
GND
4
LBD
8
N.C.
ALL DIODES IN4148
1 :3 :3
220μH PRIMARY
14 x 8mm POT CORE
C
X
3
330μF
25V
GND
1
Figure 3. +5V to ±15V Converter
cally varying from -13.6V to -14.4V as the +15V load
current changes from no load to 20mA.
Typical Applications
+5V to +15V DC-DC Converter
Figure 1 shows a simple circuit that generates +15V at
approximately 20mA from a +5V input. The MAX630
has a ±1.5% reference accuracy, so the output voltage
has an untrimmed accuracy of ±3.5% if R1 and R2 are
1% resistors. Other output voltages can also be select-
ed by changing the feedback resistors. Capacitor C
X
sets the oscillator frequency (47pF = 40kHz), while C1
limits output ripple to about 50mV.
With a low-cost molded inductor, the circuit’s efficiency
is about 75%, but an inductor with lower series resis-
tance such as the Dale TE3Q4TA increases efficiency
to around 85%. A key to high efficiency is that the
MAX630 itself is powered from the +15V output. This
provides the onboard N-channel output device with 15V
gate drive, lowering its on-resistance to about 4Ω.
When +5V power is first applied, current flows through
L1 and D1, supplying the MAX630 with 4.4V for startup.
2.5W, 3V to 5V DC-DC Converter
Some systems, although battery powered, need high
currents for short periods, and then shut down to a low-
power state. The extra circuitry of Figure 4 is designed to
meet these high-current needs. Operating in the buck-
boost or flyback mode, the circuit converts -3V to +5V.
The left side of Figure 4 is similar to Figure 1 and sup-
plies 15V for the gate drive of the external power MOS-
FET. This 15V gate drive ensures that the external device
is completely turned on and has low on-resistance.
The right side of Figure 4 is a -3V to +5V buck-boost
converter. This circuit has the advantage that when the
MAX630 is turned off, the output voltage falls to 0V,
unlike the standard boost circuit, where the output volt-
age is V
BATT
- 0.6V when the converter is shut down.
When shut down, this circuit uses less than 10µA, with
most of the current being the leakage current of the
power MOSFET.
The inductor and output-filter capacitor values have
been selected to accommodate the increased power
levels. With the values indicated, this circuit can supply
up to 500mA at 5V, with 85% efficiency. Since the left
side of the circuit powers only the right-hand MAX630,
the circuit starts up with battery voltages as low as
1.5V, independent of the loading on the +5V output.
+5V to ±15V DC-DC Converter
The circuit in Figure 3 is similar to that of Figure 1
except that two more windings are added to the induc-
tor. The 1408 (14mm x 8mm) pot core specified is an
IEC standard size available from many manufacturers
(see Table 1). The -15V output is semiregulated, typi-
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9
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