Power-Good Output (PGOOD)

The PGOOD window comparator continuously monitors

the output. PGOOD is actively held low in shutdown,

standby, and soft-start. After digital soft-start terminates,

PGOOD is released if the output is within 10% of the

nominal output voltage setting. Note that the PGOOD

window detector is completely independent of the over-

voltage and undervoltage protection fault detectors.

Output Overvoltage Protection

OVP controls the output overvoltage protection func-

tection. If overvoltage protection is enabled, the output

is continuously monitored. If the output exceeds the

overvoltage protection threshold, overvoltage protec-

tion is triggered and the DL low-side gate-driver output

is forced high. This turns on the low-side MOSFET

switch to rapidly discharge the output capacitor and

reduce the output voltage.

If LATCH is high, normal operation resumes when the

overvoltage condition ends. If LATCH is low, the DL

gate-driver output remains high until OVP is brought

When the condition that caused the overvoltage per-

sists (such as a shorted high-side MOSFET), the bat-

tery fuse will open. If overvoltage protection is enabled,

DL is kept high continuously in shutdown mode or

when UVLO is active.

Note that forcing DL high in shutdown causes the out-

put voltage to go slightly negative when energy has

been previously stored in the LC tank circuit (see the

shutdown waveforms in the Typical Operating

Characteristics). If the load cannot tolerate being

forced to a negative voltage, it may be desirable to

place a power Schottky diode across the output to act

as a reverse-polarity clamp.

Output Undervoltage Protection

UVP controls the output undervoltage protection func-

tion. Connect UVP to GND to disable undervoltage pro-

tection. The output undervoltage protection function is

similar to foldback current limiting but employs a timer

and latch rather than a variable current limit. If the output

voltage is below the undervoltage protection threshold

after the output undervoltage protection blanking time

has elapsed, the PWM is latched off and does not restart

brought low.

trip threshold of 70% of nominal. To select a different

threshold, drive UVP to a voltage between 0.4V and 1V

for a threshold between 40% and 100% of nominal.

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________

15

0.2V

2V

OUT

FB

FIXED

1.8V

TO ERROR AMP

FIXED

2.5V

MAX1844

Figure 6. Feedback Mux

DL

GND

OUT

CS

DH

FB

R1

R2

MAX1844