Rainbow Electronics MAX17008 User Manual

Page 21

Soft-Start

Soft-start begins when EN is driven high and REF is in
regulation. During soft-start, the output is ramped up
from 0V to the final set voltage at 1.3mV/μs slew rate for
SMPS1, and 0.65mV/μs for SMPS2, reducing the inrush
current and providing a predictable ramp-up time for
power sequencing:

The soft-start circuitry does not use a variable current
limit, so full output current is available immediately. The
respective PGOOD becomes high impedance approxi-
mately 200μs after the target voltage has been
reached. The MAX17007A/MAX17008 automatically
use pulse-skipping mode during soft-start and use
forced-PWM mode during soft-shutdown, regardless of
the SKIP configuration.

For automatic startup, the battery voltage should be
present before V

. If the controller attempts to bring

the output into regulation without the battery voltage
present, the fault latch trips. The controller remains shut
down until the fault latch is cleared by toggling EN or
cycling the V

power supply below 0.5V.

Soft-Shutdown

Soft-shutdown begins when the system pulls EN low, an
output undervoltage fault, or a thermal fault. During
soft-shutdown, the respective PGOOD is pulled low
immediately and the output voltage ramps down with
the same startup slew rate for the respective outputs.
After the controller reaches the 0V target, the drivers
are disabled (DL_ and DH_ pulled low) and the internal
10

Ω discharge on CSL_ activated. The MAX17007A/

MAX17008 shut down completely when both EN are
low—the reference turns off after both SMPSs have
reached the 0V target, and the supply current drops to
about 1μA (max).

Slowly discharging the output capacitors by slewing the
output over a long period of time (typically 0.5ms to
2ms) keeps the average negative inductor current low
(damped response), thereby preventing the negative
output-voltage excursion that occurs when the con-
troller discharges the output quickly by permanently
turning on the low-side MOSFET (underdamped

response). This eliminates the need for the Schottky
diode normally connected between the output and
ground to clamp the negative output-voltage excursion.

Modes of Operation

Forced-PWM Mode (

KIIP

= 5V)

The low-noise forced-PWM mode (SKIP = 5V) disables
the zero-crossing comparator, which controls the low-
side switch on-time. This forces the low-side gate-drive
waveform to constantly be the complement of the high-
side gate-drive waveform, so the inductor current
reverses at light loads while DH maintains a duty factor
of V

OUT

. The benefit of forced-PWM mode is to

keep the switching frequency fairly constant. However,
forced-PWM operation comes at a cost: the no-load 5V
bias current remains between 2mA to 5mA, depending
on the switching frequency.

The MAX17007A/MAX17008 automatically use forced-
PWM operation during shutdown, regardless of the
SKIP configuration.

Automatic Pulse-Skipping Mode

(

KIIP

= GND or 2V)

In skip mode (SKIP = GND or 2V), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is affected by a comparator that truncates
the low-side switch on-time at the inductor current’s
zero crossing. The zero-crossing comparator threshold
is set by the differential across CSL_ and CSH_.

DC output-accuracy specifications refer to the threshold of
the error comparator. When the inductor is in continuous
conduction, the MAX17007A/MAX17008 regulate the
valley of the output ripple, so the actual DC output volt-
age is higher than the trip level by 50% of the output rip-
ple voltage. In discontinuous conduction (SKIP = GND
or 2V and I

OUT

< I

LOAD(SKIP)

), the output voltage has a

DC regulation level higher than the error-comparator
threshold by approximately 1.5% due to slope compen-
sation. However, the internal integrator corrects for most
of it, resulting in very little load regulation.

When SKIP = 2V, the MAX17007A/MAX17008 use forced-
PWM operation during all dynamic output-voltage transi-
tions until 100μs after the transition has been
completed—REFIN1 and the internal target are within
±50mV (typ) and an error-amplifier transition is detected.
Since SMPS2 does not support dynamic transitions, SKIP
= 2V and SKIP = GND have the same pulse-skipping
behavior without any forced-PWM transitions.

mV μs

START

SHDN

0 65

mV μs

START

SHDN

REFIN

1 3

MAX17007A/MAX17008

Dual and Combinable QPWM Graphics

Core Controllers for Notebook Computers

______________________________________________________________________________________