MAX1630–MAX1635
Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
16 ______________________________________________________________________________________
Discharging the output capacitor through the main
inductor causes the output to momentarily go below
GND. Clamp this negative pulse with a back-biased 1A
Schottky diode across the output capacitor (Figure 1).
To ensure overvoltage protection on initial power-up,
connect signal diodes from both output voltages to VL
(cathodes to VL) to eliminate the VL power-up delay.
This circuitry protects the load from accidental overvolt-
age caused by a short-circuit across the high-side
power MOSFETs. This scheme relies on the presence
of a fuse, in series with the battery, which is blown by
the resulting crowbar current. Note that the overvoltage
circuitry will interfere with external keep-alive supplies
that hold up the outputs (such as lithium backup or hot-
swap power supplies); in such cases, the MAX1633,
MAX1634, or MAX1635 should be used.
Low-Noise Operation (PWM Mode)
PWM mode (SKIP = high) minimizes RF and audio
interference in noise-sensitive applications (such as hi-
fi multimedia-equipped systems), cellular phones, RF
communicating computers, and electromagnetic pen-
entry systems. See the summary of operating modes in
Table 2. SKIP can be driven from an external logic
signal.
Interference due to switching noise is reduced in PWM
mode by ensuring a constant switching frequency, thus
concentrating the emissions at a known frequency out-
side the system audio or IF bands. Choose an oscillator
frequency for which switching frequency harmonics
don’t overlap a sensitive frequency band. If necessary,
synchronize the oscillator to a tight-tolerance external
clock generator. To extend the output-voltage-regula-
tion range, constant operating frequency is not main-
tained under overload or dropout conditions (see
Overload and Dropout Operation section.)
PWM mode (SKIP = high) forces two changes upon the
PWM controllers. First, it disables the minimum-current
comparator, ensuring fixed-frequency operation.
Second, it changes the detection threshold for reverse-
current limit from 0mV to -100mV, allowing the inductor
current to reverse at light loads. This results in fixed-
frequency operation and continuous inductor-current
flow. This eliminates discontinuous-mode inductor ring-
ing and improves cross regulation of transformer-
coupled multiple-output supplies, particularly in circuits
that don’t use additional secondary regulation via
SECFB or V
DD
.
In most applications, tie SKIP to GND to minimize qui-
escent supply current. VL supply current with SKIP high
is typically 20mA, depending on external MOSFET gate
capacitance and switching losses.
Internal Digital Soft-Start Circuit
Soft-start allows a gradual increase of the internal cur-
rent-limit level at start-up to reduce input surge currents.
Both SMPSs contain internal digital soft-start circuits,
each controlled by a counter, a digital-to-analog con-
verter (DAC), and a current-limit comparator. In shut-
down or standby mode, the soft-start counter is reset to
zero. When an SMPS is enabled, its counter starts
counting oscillator pulses, and the DAC begins incre-
menting the comparison voltage applied to the current-
limit comparator. The DAC output increases from 0mV to
100mV in five equal steps as the count increases to 512
clocks. As a result, the main output capacitor charges
up relatively slowly. The exact time of the output rise
depends on output capacitance and load current, and
is typically 1ms with a 300kHz oscillator.
Dropout Operation
Dropout (low input-output differential operation) is
enhanced by stretching the clock pulse width to
increase the maximum duty factor. The algorithm fol-
lows: If the output voltage (V
OUT
) drops out of regula-
tion without the current limit having been reached, the
SMPS skips an off-time period (extending the on-time).
At the end of the cycle, if the output is still out of regula-
tion, the SMPS skips another off-time period. This
action can continue until three off-time periods are
skipped, effectively dividing the clock frequency by as
much as four.
The typical PWM minimum off-time is 300ns, regardless
of the operating frequency. Lowering the operating fre-
quency raises the maximum duty factor above 98%.
Adjustable-Output Feedback
(Dual Mode FB)
Fixed, preset output voltages are selected when FB_ is
connected to ground. Adjusting the main output volt-
age with external resistors is simple for any of the
MAX1630 family ICs, through resistor dividers connect-
ed to FB3 and FB5 (Figure 2). Calculate the output volt-
age with the following formula:
V
OUT
= V
REF
(1 + R1 / R2)
where V
REF
= 2.5V nominal.
The nominal output should be set approximately 1% or
2% high to make up for the MAX1630’s -2% typical
load-regulation error. For example, if designing for a
3.0V output, use a resistor ratio that results in a nominal
output voltage of 3.05V. This slight offsetting gives the
best possible accuracy. Recommended normal values
for R2 range from 5kΩ to 100kΩ. To achieve a 2.5V
nominal output, simply connect FB_ directly to CSL_.
