FAN5236 — Dual Mobile-Friendly DDR / Dual-Output PWM Controller
Circuit Description
Overview
The FAN5236 is a multi-mode, dual-channel PWM
controller intended for graphic chipset, SDRAM, DDR
DRAM, or other low-voltage power applications in
modern notebook, desktop, and sub-notebook PCs.
The IC integrates control circuitry for two synchronous
buck converters. The output voltage of each controller
can be set in the range of 0.9V to 5.5V by an external
resistor divider.
The two synchronous buck converters can operate from
either an unregulated DC source (such as a notebook
battery), with voltage ranging from 5.0V to 24V, or from
a regulated system rail of 3.3V to 5.0V. In either mode,
the IC is biased from a +5V source. The PWM
modulators use an average-current-mode control with
input voltage feedforward for simplified feedback loop
compensation and improved line regulation. Both PWM
controllers have integrated feedback loop compensation
that reduces the external components needed.
Depending on the load level, the converters can
operate in fixed-frequency PWM Mode or in a Hysteretic
Mode. Switch-over from PWM to Hysteretic Mode
improves the converters’ efficiency at light loads and
prolongs battery run time. In Hysteretic Mode,
comparators are synchronized to the main clock, which
allows seamless transition between the modes and
reduces channel-to-channel interaction. The Hysteretic
Mode can be inhibited independently for each channel if
variable frequency operation is not desired.
The FAN5236 can be configured to operate as a
complete DDR solution. When the DDR pin is set HIGH,
the second channel provides the capability to track the
output voltage of the first channel. The PWM2 converter
is prevented from going into Hysteretic Mode if the DDR
pin is set HIGH. In DDR Mode, a buffered reference
voltage (buffered voltage of the REF2 pin), required by
DDR memory chips, is provided by the PG2 pin.
When V
IN
is from the battery, it’s typically higher than
7.5V. As shown in Figure 7, 180° operation is
undesirable because the turn-on of the V
DDQ
converter
occurs very near the decision point of the V
TT
converter.
CLK
V
D DQ
V
TT
Figure 7. Noise-Susceptible 180° Phasing for DDR1
In-phase operation is optimal to reduce inter-converter
interference when V
IN
is higher than 5V (when V
IN
is
from a battery), as shown in Figure 8. Because the duty
cycle of PWM1 (generating V
DDQ
) is short, the switching
point occurs far away from the decision point for the V
TT
regulator, whose duty cycle is nominally 50%.
CLK
V
DDQ
V
TT
Figure 8. Optimal In-Phase Operation for DDR1
When V
IN
≈
5V, 180° phase-shifted operation can be
rejected for the reasons demonstrated in Figure 7.
In-phase operation with V
IN
≈
5V is even worse, since
the switch point of either converter occurs near the
switch point of the other converter, as seen in Figure 9.
In this case, as V
IN
is a little higher than 5V, it tends to
cause early termination of the V
TT
pulse width.
Conversely, the V
TT
switch point can cause early
termination of the V
DDQ
pulse width when V
IN
is slightly
lower than 5V.
CLK
V
DDQ
V
TT
Converter Modes and Synchronization
Table 3. Converter Modes and Synchronization
Mode
DDR1
DDR2
DUAL
V
IN
Battery
+5V
ANY
VIN Pin
V
IN
R to GND
V
IN
DDR
Pin
HIGH
HIGH
LOW
PWM 2 w.r.t.
PWM1
IN PHASE
+90°
+180°
Figure 9.
Noise-Susceptible In-Phase Operation
for DDR2
When used as a dual converter, as shown in Figure 6,
out-of-phase operation with 180-degree phase shift
reduces input current ripple.
For “two-step” conversion (where the V
TT
is converted
from V
DDQ
as in Figure 5) used in DDR Mode, the duty
cycle of the second converter is nominally 50% and the
optimal phasing depends on V
IN
. The objective is to
keep noise generated from the switching transition in
one converter from influencing the "decision" to switch
in the other converter.
© 2002 Fairchild Semiconductor Corporation
FAN5236 • Rev. 1.3.2
10
These problems are solved by delaying the second
converter’s clock by 90°, as shown in Figure 10. In this
way, all switching transitions in one converter take place
far away from the decision points of the other converter.
CLK
V
DDQ
V
TT
Figure 10. Optimal 90° Phasing for DDR2
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