MAX1632AEAI Datasheet, PDF (10/29 Pages) Maxim Integrated Products – Multi-Output, Low-Noise Power-Supply Controllers for Notebook Computers

Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
_______________Detailed Description
The MAX1630A is a dual, BiCMOS, switch-mode power-
supply controller designed primarily for buck-topology
regulators in battery-powered applications where high effi-
ciency and low quiescent supply current are critical. Light-
load efficiency is enhanced by automatic Idle Mode
operation, a variable-frequency pulse-skipping mode that
reduces transition and gate-charge losses. Each step-
down, power-switching circuit consists of two n-channel
MOSFETs, a rectifier, and an LC output filter. The output
voltage is the average AC voltage at the switching node,
which is regulated by changing the duty cycle of the
MOSFET switches. The gate-drive signal to the n-channel
high-side MOSFET must exceed the battery voltage, and
is provided by a flying-capacitor boost circuit that uses a
100nF capacitor connected to BST_.
Devices in the MAX1630A family contain 10 major circuit
blocks (Figure 2).
The two PWM controllers each consist of a Dual Mode
feedback network and multiplexer, a multi-input PWM
comparator, high-side and low-side gate drivers, and
logic. The MAX1630A/MAX1631A/MAX1632A contain
fault-protection circuits that monitor the main PWM out-
puts for undervoltage and overvoltage. A power-on
sequence block controls the power-up timing of the
main PWMs and determines whether one or both of the
outputs are monitored for undervoltage faults. The
MAX1630A/MAX1632A/MAX1633A/MAX1635A include
a secondary feedback network and 12V linear regulator
to generate a 12V output from a coupled-inductor fly-
back winding. The MAX1631A/MAX1634A have an
SECFB instead, which allows a quasi-regulated,
adjustable-output, coupled-inductor flyback winding to
be attached to either the 3.3V or the 5V main inductor.
Bias generator blocks include the 5V IC internal rail (VL)
linear regulator, 2.5V precision reference, and automatic
bootstrap switchover circuit. The PWMs share a com-
mon 200kHz/300kHz synchronizable oscillator.
These internal IC blocks are not powered directly from
the battery. Instead, the 5V VL linear regulator steps
down the battery voltage to supply both VL and the
gate drivers. The synchronous-switch gate drivers are
directly powered from VL, while the high-side switch
gate drivers are indirectly powered from VL through an
external diode-capacitor boost circuit. An automatic
bootstrap circuit turns off the +5V linear regulator and
powers the IC from the 5V PWM output voltage if the
output is above 4.5V.
PWM Controller Block
The two PWM controllers are nearly identical. The only
differences are fixed output settings (3.3V vs. 5V), the
VL/CSL5 bootstrap switch connected to the +5V PWM,
and SECFB. The heart of each current-mode PWM con-
troller is a multi-input, open-loop comparator that sums
three signals: the output voltage error signal with
respect to the reference voltage, the current-sense sig-
nal, and the slope compensation ramp (Figure 3). The
PWM controller is a direct-summing type, lacking a tra-
ditional error amplifier and the phase shift associated
with it. This direct-summing configuration approaches
ideal cycle-by-cycle control over the output voltage.
When SKIP = low, Idle Mode circuitry automatically
optimizes efficiency throughout the load current range.
Idle Mode dramatically improves light-load efficiency
by reducing the effective frequency, which reduces
switching losses. It keeps the peak inductor current
above 25% of the full current limit in an active cycle,
allowing subsequent cycles to be skipped. Idle Mode
transitions seamlessly to fixed-frequency PWM opera-
tion as load current increases.
With SKIP = high, the controller always operates in
fixed-frequency PWM mode for lowest noise. Each
pulse from the oscillator sets the main PWM latch that
turns on the high-side switch for a period determined
by the duty factor (approximately V_OUT/V_IN). As the
high-side switch turns off, the synchronous rectifier
latch sets; 60ns later, the low-side switch turns on. The
low-side switch stays on until the beginning of the next
clock cycle.
In PWM mode, the controller operates as a fixed-
frequency current-mode controller where the duty ratio
is set by the input/output voltage ratio. The current-
mode feedback system regulates the peak inductor
Table 3. SKIP PWM Table
SKIP
LOAD
CURRENT
MODE
DESCRIPTION
Pulse-skipping, supply cur-
Low
Light
Idle
rent = 250µA at V_IN = 12V,
discontinuous inductor
current
Low
Heavy
PWM
Constant-frequency PWM,
continuous inductor current
High
High
Light
Heavy
PWM
PWM
Constant-frequency PWM,
continuous inductor current
Constant-frequency PWM,
continuous inductor current
10 ______________________________________________________________________________________