Rainbow Electronics MAX1636 - Manual (Page 15)

Output Overvoltage Protection (OVP)
The overvoltage crowbar protection circuit is intended
to blow a fuse in series with the battery if the main
SMPS output rises significantly higher than its preset
level. In normal operation, the output is compared to
the internal precision reference voltage. If the output
goes 7% above nominal, the synchronous rectifier
MOSFET turns on 100% (the high-side MOSFET is
simultaneously forced off) in order to draw massive
amounts of battery current to blow the fuse. This safety
feature does not protect the system against a failure of
the controller IC itself but is intended primarily to guard
against a short across the high-side MOSFET. A crow-
bar event is latched and can only be reset by a rising
edge on SHDN (or by removal of the V+ supply volt-
age). The overvoltage-detection decision is made rela-
tive to the regulation point.
The overvoltage comparators are kept inactive in
standby mode. Instead, the DL driver is simply left in
the high state. However, DL does not turn on until the
output has decayed to less than 1V. This prevents con-
flicts in systems where the output is held up by an
external source in suspend or backup mode. The OVP
pin has an internal pulldown resistor that is only turned
on during the reset phase. The OVP pin’s state is then
sampled and stored internally. A floating OVP pin
implies no overvoltage protection.
Boost High-Side Gate-Drive Supply (BST)
Gate-drive voltage for the high-side N-channel switch is
generated by a flying-capacitor boost circuit (Figure 2).
The capacitor between BST and LX is alternately
charged from the VL supply and placed parallel to the
high-side MOSFET's gate-source terminals.
On start-up, the synchronous rectifier (low-side MOS-
FET) forces LX to 0V and charges the boost capacitor
to 5V. On the second half-cycle, the SMPS turns on the
high-side MOSFET by closing an internal switch
between BST and DH. This provides the necessary
enhancement voltage to turn on the high-side switch,
an action that boosts the 5V gate-drive signal above
the battery voltage.
MAX1636
Low-Voltage, Precision Step-Down
Controller for Portable CPU Power
______________________________________________________________________________________ 15
MODE S SH HD DN N OVP HOW ENTERED STATUS NOTES
Output
UVLO
High
Run High High
All circuit blocks
active
Normal operation
Don’t care
V OUT < 70% of
nominal after
20–30ms timeout
expires
Standby Low High
VL = on
REF = off
DL = high
RESET = high-Z
(high state)
DL = high to enforce overvoltage
protection
VL = on
REF = off
DL = low
RESET = low
Cycling SHDN or a power-on reset exits
output UVLO.
Shutdown Low Low
All circuit blocks
inactive
Lowest possible quiescent consumption
Thermal
Shutdown
High
Overvoltage
(crowbar)
High High V OUT > 7% too high
VL = on
REF = off
DL = high
RESET = low
Cycling SHDN or a power-on reset exits
crowbar.
High T
J
> +150°C
VL = on
REF = off
DL = high
RESET = low
Cycling SHDN or a power-on reset exits
thermal shutdown.
Thermal
Shutdown
High Low T J > +150°C
All circuit blocks
inactive Cycling SHDN or a power-on reset exits
thermal shutdown.
Table 5. Operating Modes