FAN5026_11 Datasheet, PDF (9/17 Pages) Fairchild Semiconductor – Dual DDR / Dual-Output PWM Controller

Circuit Description
Overview
The FAN5026 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
an unregulated DC source (such as a notebook
battery), with voltage ranging from 5.0V to 16V, 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.
The FAN5026 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 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.
Converter Modes and Synchronization
Table 3. Converter Modes and Synchronization
Mode
DDR1
DDR2
DUAL
V_IN
Battery
+5V
ANY
VIN
Pin
VIN
R to
GND
VIN
DDR
Pin
HIGH
HIGH
LOW
PWM 2
w.r.t.
PWM1
IN PHASE
+90°
+180°
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 VTT is converted
from VDDQ as in Figure 5) used in DDR Mode, the duty
cycle of the second converter is nominally 50% and the
optimal phasing depends on VIN. The objective is to
keep noise generated from the switching transition in
one converter from influencing the "decision" to switch
in the other converter.
When VIN 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 VDDQ converter
occurs very near the decision point of the VTT converter.
CL K
V_{D DQ}
VTT
Figure 7. Noise-Susceptible 180° Phasing
for DDR1
In-phase operation is optimal to reduce inter-converter
interference when VIN is higher than 5V, (when VIN is
from a battery), as shown in Figure 8. Because the duty
cycle of PWM1 (generating VDDQ) is short, the switching
point occurs far away from the decision point for the VTT
regulator, whose duty cycle is nominally 50%.
CLK
V_DDQ
V_TT
Figure 8. Optimal In-Phase Operation for DDR1
When VIN ≈ 5V, 180° phase-shifted operation can be
rejected for the reasons demonstrated in Figure 7.
In-phase operation with VIN ≈ 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 VIN is a little higher than 5V, it tends to
cause early termination of the VTT pulse width.
Conversely, the VTT switch point can cause early
termination of the VDDQ pulse width when VIN is slightly
lower than 5V.
CLK
V_DDQ
V_TT
Figure 9.
Noise-Susceptible In-Phase Operation
for DDR2
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.
CL K
V_DDQ
V_TT
Figure 10. Optimal 90° Phasing for DDR2
© 2005 Fairchild Semiconductor Corporation
FAN5026 • Rev. 1.0.8
9
www.fairchildsemi.com