Mu lt i-Ou t p u t , Lo w -No is e P o w e r-S u p p ly
Co n t ro lle rs fo r No t e b o o k Co m p u t e rs
0–MAX1635
S h u t d o w n Mo d e
Holding SHDN low puts the IC into its 4µA shutdown
mode. SHDN is logic input with a threshold of about 1V
Ou t p u t Un d e rvo lt a g e S h u t d o w n P ro t e c t io n
(MAX1 6 3 0 /MAX1 6 3 1 /MAX1 6 3 2 )
The output undervoltage lockout circuit is similar to
foldback current limiting, but employs a timer rather
than a variable current limit. Each SMPS has an under-
voltage protection circuit that is activated 6144 clock
cycles after the SMPS is enabled. If either SMPS output
is under 70% of the nominal value, both SMPSs are
latched off and their outputs are clamped to ground by
the s ync hronous re c tifie r MOSFETs (s e e Outp ut
Overvoltage Protection section). They won’t restart until
SHDN or RUN/ON3 is toggled, or until V+ power is
cycled below 1V. Note that undervoltage protection can
make prototype troubleshooting difficult, since you
have only 20ms or 30ms to figure out what might be
wrong with the circuit before both SMPSs are latched
off. In extreme cases, it may be useful to substitute the
MAX1633/MAX1634/MAX1635 into the p rototyp e
breadboard until the prototype is working properly.
(the V of an internal N-channel MOSFET). For auto-
TH
matic start-up, bypass SHDN to GND with a 0.01µF
capacitor and connect it to V+ through a 220kΩ resistor.
P o w e r-Up S e q u e n c in g
a n d ON/OFF Co n t ro ls
Start-up is controlled by RUN/ON3 and TIME/ON5 in
conjunction with SEQ. With SEQ tied to REF, the two
control inputs act as separate ON/OFF controls for
each supply. With SEQ tied to VL or GND, RUN/ON3
b e c ome s the ma s te r ON/OFF c ontrol inp ut a nd
TIME/ON5 b e c ome s a timing p in, with the d e la y
between the two supplies determined by an external
capacitor. The delay is approximately 800µs/nF. The
+3.3V supply powers-up first if SEQ is tied to VL, and
the +5V supply is first if SEQ is tied to GND. When driv-
ing TIME/ON5 as a control input with external logic,
always place a resistor (>1kΩ) in series with the input.
This prevents possible crowbar current due to the inter-
nal discharge pull-down transistor, which turns on in
standby mode and momentarily at the first power-up or
in shutdown mode.
Ou t p u t Ove rvo lt a g e P ro t e c t io n
(MAX1 6 3 0 /MAX1 6 3 1 /MAX1 6 3 2 )
Both SMPS outputs are monitored for overvoltage. If
either output is more than 7% above the nominal regu-
la tion p oint, b oth low-s id e g a te d rive rs (DL_) a re
latched high until SHDN or RUN/ON3 is toggled, or until
V+ power is cycled below 1V. This action turns on the
synchronous rectifiers with 100% duty, in turn rapidly
discharging the output capacitors and forcing both
SMPS outputs to ground. The DL outputs are also kept
high whenever the corresponding SMPS is disabled,
and in shutdown if VL is sustained.
RESET P o w e r-Go o d Vo lt a g e Mo n it o r
The power-good monitor generates a system RESET sig-
nal. At first power-up, RESET is held low until both the
3.3V and 5V SMPS outputs are in regulation. At this point,
an internal timer begins counting oscillator pulses, and
RESET continues to be held low until 32,000 cycles have
elapsed. After this timeout period (107ms at 300kHz or
160ms at 200kHz), RESET is actively pulled up to VL. If
SEQ is tied to REF (for separate ON3/ON5 controls), only
the 3.3V SMPS is monitored—the 5V SMPS is ignored.
Table 4. Operating Modes
SEQ
X
RUN/ON3
X
TIME/ON5
X
MODE
DESCRIPTION
SHDN
Low
Shutdown
Standby
Run
All circuit blocks turned off. Supply current = 4µA.
Both SMPSs off. Supply current = 30µA.
3.3V SMPS enabled/5V off
High
High
High
High
High
High
High
High
Ref
Ref
Ref
Ref
GND
GND
VL
Low
Low
High
Low
Low
High
Run
5V SMPS enabled/3.3V off
High
Low
High
Run
Both SMPSs enabled
Timing capacitor
Timing capacitor
Timing capacitor
Timing capacitor
Standby
Run
Both SMPSs off. Supply current = 30µA.
Both SMPSs enabled. 5V enabled before 3.3V.
Both SMPSs off. Supply current = 30µA.
Both SMPSs enabled. 3.3V enabled before 5V.
High
Low
Standby
Run
VL
High
X = Don’t Care
______________________________________________________________________________________ 15
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
