FAN5233 Datasheet, PDF (10/15 Pages) Fairchild Semiconductor – System Electronics Regulator for Mobile PCs

FAN5233
PRODUCT SPECIFICATION
(for the duration of the contact) to power the 5V-ALWAYS
long enough for the µC to power up and in turn latch the
SDWN pin high.
Once the SDWN is high then the Main Regulator voltages
are en-abled to go high if the respective SDN3.3 and SDN5
go high.
MAIN 3.3V and 5V Softstart, Sequencing and
Stand-by
Softstart of the 3.3V and 5V converters is accomplished by
means of an external capacitor between pins SDN3.3 (SDN5)
and ground.
The 3.3V (5V) main converter is turned ON if SDWN and
SDN3.3 (SDN5) are both high and is turned off if either SDWN
or SDN3.3 (SDN5) is low.
Stand-by mode is defined as the condition by which V-Mains
are OFF and V-ALWAYS are ON (SDWN=1 and
SDN3.3=SDN5=0).
Forced PWM Mode
The controller can be forced to stay in PWM mode under any
load conditions by taking FPWM high. It is recommended that
during power-up FPWM be driven high to ensure a more robust
regulator turn-on and to provide a control over the output
capacitor inrush current by limiting the maximum duty cycle.
ALWAYS mode of Operation
If it is desired that 5V-ALWAYS is always ON then the SDWN
pin must be connected to Vin permanently. This way the
ALWAYS regulator comes up as soon as there is power while
the state of the Main regulators can be controlled via the SDN5
and SDN3.3 pins.
Sequencing Table
SDN5
X
0
1
0
1
SDN3.3
X
0
0
1
1
SDWN
0
1
1
1
1
5V
ALWAYS
0
1
1
1
1
5V
MAIN
0
0
1
0
1
3.3V
MAIN
0
0
0
1
1
3.3V and 5V Light Load Mode
The 3.3V and 5V converters are synchronous bucks, and can
operate in two quadrants, this means that the ripple current is
constant and independent of the load current. At light loads,
this ripple current translates into poor efficiency, since it
causes circulating current losses in the MOSFETs. To opti-
mize the efficiency at light loads, then, the FAN5233
switches from normal operation to a special light load mode
after an 8 clock pulse delay. This prevents false triggering
when the voltage across the on-state low-side MOSFET goes
positive. Vice-versa when this voltage becomes negative the
FAN5233 switches back to PWM operation. The current
threshold for switch to and from light load is therefore:
Ith = Iripplepeak
In light load mode, the FAN5233 switches from PWM (pulse
width modulation) to PFM (pulse frequency modulation),
which reduces the gate drive current. Transition to the RFM
mode can be inhibited by pulling the FPWM pin high.
As the load current becomes very light, the FAN5233 begins
pulse skipping, but remains synchronized with the clock. See
next section for low side drive management.
Low Side Driver Forcing in Light Load
During light load operation, the Low Side Driver (LSD) is
traditionally turned permanently OFF to avoid current inver-
sion in the inductor and associated efficiency losses. At the
same time the low side driver also needs to be turned ON in
order to a) measure current (current is sensed on the low side
driver) and b) assure proper operation of the charge pump,
especially under low current and low input voltage condi-
tions. In order to accomplish all the above, when the circuit
enters hysteretic operation the LSD is kept “ON” to re-circu-
late positive and decaying currents (corresponding to nega-
tive drops across low side driver Rdson) and turned off as
soon as current crosses zero (corresponding to drop across
Rdson becoming positive). This way the low side driver is
utilized in “partial duty” or as “active zero drop diode”
(compared to classic light load operation in which the LSD is
turned permanently OFF) allowing more functionality with-
out loss in efficiency.
3.3V Voltage Adjustment
The output voltage of the 3.3V converter can be increased by
as much as 10% by inserting a resistor divider in the feedback
line. The feedback impedance is about 66KΩ. Thus, for exam-
ple, to increase the output of the 3.3V by 10%, use a 2.21KΩ/
33.2KΩ divider. Note that the output of the 5V regulator can-
not be adjusted. The feedback line of the 5V regulator is used
internally as a 5V supply and, therefore, cannot tolerate any
impedance in series with it.
3.3V and 5V Main Overvoltage Protection
(Soft Crowbar)
When the output voltage of the 3.3V (or the 5V) converter
exceeds approximately 115% of nominal, the converter enters
the over-voltage (OV) protection mode, with the goal of pro-
tecting the load from damage. During operation, severe load
dump or a short of an upper MOSFET could cause the output
voltage to increase significantly over normal operation range
without circuit protection. When the output exceeds the over-
voltage threshold, the over-voltage comparator forces the
lower gate driver high and turns the lower MOSFET on. This
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REV. 1.0.6 1/22/02