FAN5235 Datasheet, PDF (7/15 Pages) Fairchild Semiconductor – System Electronics Regulator for Mobile PCs

3.3V ALWAYS
VIN
LDO
LDO
FAN5235
5V/3.3V-ALWAYS
FAN5235
5V ALWAYS
VFB5
Figure 3. FAN5235 5V/3.3V—ALWAYS Internal Block Diagram
Functional Description
The FAN5235 is a high efficiency and high precision DC/DC
controller for notebook and other portable applications. It
provides all of the voltages necessary for system electronics:
5V, 3.3V, 12V, and both 3.3V-ALWAYS and 5V-ALWAYS.
Utilization of both input and output voltage feedback in a
current-mode control allows for fast loop response over a
wide range of input and output variations. Current sense
based on MOSFET R_DS,on gives maximum efficiency, while
also permitting the use of a sense resistor for high accuracy.
3.3V and 5V Architecture
The 3.3V and 5V switching regulator outputs of the
FAN5235 are generated from the unregulated input voltage
using synchronous buck converters. Both high side and low-
side MOSFETs are N-channel.
The 3.3V and 5V switchers have pins for current sensing and
for setting of output over-current threshold using MOSFET
R_DS,on. Each converter has a pin for voltage-sense feedback,
a pin that shuts down the converter, and a pin for generating
the boost voltage to drive the high-side MOSFET.
The following discussion of the FAN5235 design will be
done with reference to Figures 1 through 4, showing the
internal block diagram of the IC.
3.3V and 5V PWM Current Sensing
Peak current sensing is done on the low side driver because
of the very low duty-cycle on the high side MOSFET. The
current is sampled 50ns after turn on and the value is held for
current feedback and over-current limit.
3.3V and 5V PWM Loop Compensation
The 3.3V and 5V control loops of the FAN5235 function as
voltage mode with current feedback for stability. They each
have an independent voltage feedback pin, as shown in
Figure 1. They use voltage feed-forward to guarantee loop
rejection of input voltage variation: that is to say that the
PWM (pulse width modulation) ramp amplitude is varied as
a function of the input voltage. Compensation of the control
loops is done entirely internally using current-mode feed-
back compensation. This scheme allows the bandwidth and
phase margin to be almost independent of output capacitance
and ESR.
3.3V and 5V PWM Current Limit
The 3.3V and 5V converters each sense the voltage across
their own low-side MOSFET to determine whether to enter
current limit. If an output current in excess of the current
limit threshold is measured then the converter enters a pulse
skipping mode where Iout is equal to the over-current (OC)
set limit. After 8 clock cycles then the regulator is latched off
(HSD and LSD off). This is the likely scenario in the 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 2µs delay.
Selection of a current-limit set resistor must include the
tolerance of the current-limit trip point, the MOSFET on
resistance and temperature coefficient, and the ripple current,
in addition to the maximum output current.
Example: Maximum DC output current on the 5V is 5A,
the MOSFET R_DS,on is 17mΩ, and the inductor is 5µH at a
current of 5A. Because of the low R_DS,on, the low-side
MOSFET will have a maximum temperature (ambient +
self-heating) of only 75°C, at which its R_DS,on increases to
20mΩ.
REV. 1.3.3 1/3/02
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