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FAN5232 Datasheet(PDF) 6 Page - Fairchild Semiconductor

Part No. FAN5232
Description  Adjustable PWM Buck Controller for LCD PCs
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN5232 Datasheet(HTML) 6 Page - Fairchild Semiconductor

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FAN5232
PRODUCT SPECIFICATION
6
REV. 1.1.1 10/7/02
Loop Compensation
The switcher regulator control loop of the FAN5232 is
current-mode with voltage feed-forward. It uses voltage
feed-forward to guarantee loop rejection of input voltage
variation: the ramp amplitude is varied as a function of the
input voltage. Compensation of the control loop is done
entirely internally using current-mode compensation. This
scheme allows the bandwidth and phase margin to be almost
independent of output capacitance and capacitors’ ESR. Use
of a current sense resistor other than the recommended 1K
may affect the converter’s stability.
Current Sensing
Current sensing is done by measuring the voltage across the
low side MOSFET 50nsec after it is turned on. This value is
then held for current feedback and over-current limit. The
gain is set by an external resistor from the drain to the ISNS
pin, which is normally set to be 1K
Ω.
Current Limit
The converter senses the voltage across its low-side
MOSFET to determine when to enter current limit. If output
current in excess of the current limit threshold is measured,
the converter enters pulse skip mode with Iout equal to the
over-current (OC) limit. If this situation persists for 8 clock
cycles then the regulator is latched off (HSD and LSD off).
This is the likely scenario in case of a “soft” short. If the
short is “hard”, it will instantly trigger the under-voltage
protection, which again will latch the regulator off (HSD and
LSD off) after a 2µsec delay.
Selection of a current-limit set resistor must include the tol-
erance of the current-limit trip point, the MOSFET resis-
tance and temperature coefficient, and the ripple current, in
addition to the maximum output current.
Example: Maximum DC output current on the 12V is 8A,
the MOSFET RDS,on is 17mΩ, and the inductor is 4.7µH
at a current of 8A. Because of the low RDS,on, the low-side
MOSFET will have a maximum temperature (ambient +
self-heating) of only 75°C, at which its RDS,on increases to
24m
Ω.
Peak current is DC output current plus peak ripple current:
where T is the maximum period, VO is output voltage, Vin is
input voltage, and L is the inductance. This current generates
a voltage on the low-side MOSFET of 11A • 24m
Ω =
254mV. The current limit threshold is typically 150mV
(worst-case 135mV) with R2 = 1K
Ω, and so this value must
be decreased to (135/254) • 1K
Ω = 531Ω.
Precision Current Limit
Precision current limiting can be achieved by placing a dis-
crete sense resistor between the source of the low-side
MOSFET and ground. Sensing is then accomplished with
the 1K
Ω resistor between the sense resistor and the IFBSW
pin, as shown in Figure 2. In this case, current limit accuracy
is set by the tolerance of the IC, ±10%.
Figure 2. Precision Current Sensing
Softstart
Softstart of the switcher is accomplished by means of an
external capacitor between pins SDNADJ and ground.
Overvoltage Protection (Soft Crowbar)
When the output voltage of the switcher exceeds approxi-
mately 115% of nominal, it enters into over-voltage (OV)
protection, with the goal of protecting the load from damage.
During operation, severe load dump or a short of an upper
MOSFET can cause the output voltage to increase signifi-
cantly over normal operation range. When the output
exceeds the over-voltage threshold of 115%, the over-voltage
comparator forces the lower gate driver high and turns the
lower MOSFET on. This will pull down the output voltage
and eventually may blow the battery fuse. As soon as output
voltage drops below the threshold, the OVP comparator is
disengaged.
This OVP scheme provides a soft crowbar function (bang-
bang control followed by blow of the fuse), which helps to
tackle severe load transients and does not invert the output
voltage when activated – common problem for OVP schemes
with a latch. The prevention of the output inversion saves the
use of a Schottky diode across the load.
Undervoltage Protection
When the output voltage of the switcher falls below 75% of
nominal value, after a 2usec delay it goes into under-voltage
protection. In under-voltage protection, the high and low side
MOSFETs are turned off. Once under-voltage protection is
triggered, it remains on until power is recycled.
5V/3.3V-ALWAYS Operation
The 5V-ALWAYS supply is generated from the on-chip
linear regulator off the input supply voltage. The
3.3V-ALWAYS is generated from a linear regulator attached
internally to the 5V-ALWAYS.
I
pk
I
DC
TV
O
V
in
V
o
()
2LV
in
--------------------------------------------
+
8A
4
µs 12V
19V
12V
()
2
4.7
µH12V
----------------------------------------------------------------11A
=
+
=
LSD
ISNS
PGND


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