Rainbow Electronics MAX1844 - Manual (Page 15)

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-
tion. Connect OVP to V
CC to disable overvoltage pro-
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
high, SHDN is toggled, or V
CC is cycled below 1V.
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
until V
CC
power is cycled. SHDN is toggled, or UVP is
brought low.
Connect UVP to V CC to enable the default undervoltage
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.
MAX1844
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
V BATT
V
OUT
R1
R2
MAX1844
Figure 7. Setting V
OUT
with a Resistor-Divider