SC480 SEMTECH [Semtech Corporation], SC480 Datasheet - Page 10

• Manufacturer Part Number: SC480
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: SEMTECH [Semtech Corporation]

+5V Bias Supplies
The SC480 requires an external +5V bias supply in addition
to the battery. If stand-alone capability is required, the
+5V supply can be generated with an external linear
regulator. To minimize crosstalk, the controller has seven
supply pins: VDDP (2 pins), PGND1 (2 pins), PGND2,
VCCA and AGND.
The controller requires its own AGND plane which should
be tied by a single trace to the negative terminal of the
output capacitor. All external components referenced
to AGND in the schematic should then be connected to
the AGND plane. The supply decoupling capacitor should
be tied between VCCA and AGND. A single 10Ω resistor
should be used to decouple the VCCA supply from the
main VDDP supply. PGND can then be a separate plane
which is not used for routing analog traces. All PGND
connections should connect directly to this plane with
special attention given to avoiding indirect connections
between AGND and PGND which will create ground loops.
As mentioned above, the AGND plane must be connected
to the PGND plane at the negative terminal of the output
capacitor. The VDDP input provides power to the upper
and lower gate drivers. A decoupling capacitor for the
VDDP supply and PGND is recommended. No series
resistor between VDDP and the 5 volt bias is required.
Pseudo-Fixed Frequency Constant On-Time
PWM Controller
The PWM control method is a constant-on-time, pseudo-
fi xed frequency PWM controller, see Figure 1. The ripple
voltage seen across the output capacitor’s ESR provides
the PWM ramp signal, eliminating the need for a current
sense resistor. The on-time is determined by a one-shot
whose period is proportional to output voltage, and
inversely proportional to input voltage. A separate one-
shot sets the minimum off-time (typically 425ns).
On-Time One-Shot (T
The on-time one-shot comparator has two inputs. One
input looks at the output voltage, while the other input
samples the input voltage and converts it to a proportional
current. This current charges an internal on-time capacitor.
The TON time is the time required for this capacitor to
charge from zero volts to VOUT, thereby making the on-
time of the high-side switch directly proportional to output
voltage and inversely proportional to input voltage. This
implementation results in a nearly constant switching
frequency without the need of a clock generator.
© 2006 Semtech Corp.
POWER MANAGEMENT
POWER MANAGEMENT
Application Information
Application Information
ON
)
10
R
the TON pin.
VDDQ/VTT Enable & Power-Save
The EN/PSV pin controls the VDDQ supply and the REF
output (1/2 of VDDQ). VTTEN enables the VTT supply. The
VTT and VDDQ supplies may be enabled independently.
When EN/PSV is tied to VCCA the VDDQ controller is
enabled in power-save mode. When the EN/PSV pin is
fl oated, an internal resistor divider activates the VDDQ
controller with power-save disabled. If PSAVE is enabled,
the SC480 PSAVE comparator looks for inductor current
to cross zero on eight consecutive cycles. Once observed,
the controller enters power-save and turns off the low-
side MOSFET when the current crosses zero. To improve
the effi ciency and add hysteresis, the on-time is increased
by 20% in power-save. The effi ciency improvement at light
loads more than offsets the disadvantage of slightly higher
output ripple. If the inductor current does not cross zero
on any switching cycle, the controller immediately exits
power-save. Since the controller counts zero crossings,
the converter can sink current as long as the current does
not cross zero on eight consecutive cycles. This allows the
output voltage to recover quickly in response to negative
load steps even when power-save is enabled.
VDDQ Voltage Selection
VDDQ voltage is set using the FB pin. Grounding FB sets
VDDQ to fi xed 2.5V. Connecting FB to +5V sets VDDQ to
fi xed 1.8V. VDDQ can also be adjusted from 1.5 to 3.0V
using external resistors, see Figure 2. The voltage at FB is
then compared to the internal 1.5V reference.
TON
T
ON
is a resistor connected between the input supply and
To SC480 FB (pin 9)
3.3x10
12
To VDDQ output capacitor
(RTON
Figure 2
R2
37x10
R3
) 3
C
V
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OUT
V
IN
SC480
50
ns