MAX17007 datasheet(21/35 Pages) MAXIM

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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 MAX17007/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 VCC. 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 VCC 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

Ω discharge on CSL_ activated. The MAX17007/

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 (S

KIIP

P = 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 VOUT/VIN. 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 MAX17007/MAX17008 automatically use forced-

PWM operation during shutdown, regardless of the

SKIP configuration.

Automatic Pulse-Skipping Mode

KIIP

P = 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 thresh-

old of the error comparator. When the inductor is in con-

tinuous conduction, the MAX17007/MAX17008 regulate

the valley of the output ripple, so the actual DC output

voltage is higher than the trip level by 50% of the output

ripple voltage. In discontinuous conduction (SKIP =

GND or 2V and IOUT < ILOAD(SKIP)), the output voltage

has a DC regulation level higher than the error-com-

parator threshold by approximately 1.5% due to slope

compensation. However, the internal integrator corrects

for most of it, resulting in very little load regulation.

When SKIP = 2V, the MAX17007/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 s

START

SHDN

065

mV s

START

SHDN

REFIN

Dual and Combinable QPWM Graphics

Core Controllers for Notebook Computers

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