FAN5235 Datasheet, PDF (10/15 Pages) Fairchild Semiconductor – System Electronics Regulator for Mobile PCs

FAN5235
Error Amplifier output voltage clamp
During a load transient the error amplifier voltage is allowed
full swing. After two clock cycles, if the amplifier is still out
of range the voltage and consequently the duty cycle (DC) is
clamped. The DC clamp automatically limits the build up of
over-currents during abnormal conditions, including short
circuits:
VFB=0.5V+Vo/8
–
EA
VREF +
VRAMP =0.5V+Vin/8
+
–
Vclamp=0.5V+
+Vo/8 +/-0.2V
0.4V
2 Cycles
Counter
Figure 6. Duty-Cycle Clamp
Thermal shutdown
If the die temperature of the FAN5235 exceeds safe limits,
the IC shuts itself off. When the over-temperature (OT)
event ends, the IC comes back to normal operation. There is
a 25°C thermal hysteresis between shutdown and start up.
Input UVLO
If the input voltage falls below the UVLO threshold, the
FAN5235 turns itself off and stays off as long as Input
voltage is below threshold.
IC Protections Table
HSD
Buck
LSD
Buck
LDO
LSD
Boost
OC/UV
(Bucks)
OFF-LATCH OFF-LATCH
ON
OFF-LATCH
OC/UV
(LDO)
"
"
OFF-LATCH
"
OV (Buck)* ^OFF
SOFT
ON
ON
CROWBAR
OV (Boost)
ON
ON
ON
OFF
SDWN=0
OFF
OFF
OFF
OFF
OT
OFF
OFF
OFF
OFF
UV (Boost) ^{OFF-LATCH OFF-LATCH}
ON
OFF-LATCH
OC (Boost) ^ON
ON
ON
33% DC
* Only the converter in Over-Voltage goes in SOFT CROW-
BAR mode.
Generic Mobile System Block Diagram
Vin=5.6 to 24V
5V
SDN5
3.3V
SDN3.3
FAN5235
µC
8051
PGOOD
5V-Always
µC
PGOOD
3.3V-Always _EN
SDWN
µP CODE EXECUTION
Vcpu
RC5231 ^1.5V
2.5V
µP
Clock
CPU
CPU
PGOOD
SDWN
RESET
LOGIC
Figure 7. System Block Diagram
10
REV. 1.3.3 1/3/02