SC488 Semtech Corporation, SC488 Datasheet - Page 9

• Manufacturer Part Number: SC488
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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+5V Bias Supplies
The SC488 requires an external +5V bias supply in addi-
tion 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 in-
versely 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 capaci-
tor. 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 volt-
age. This implementation results in a nearly constant
switching frequency without the need of a clock generator.
POWER MANAGEMENT
Application Information
Application Information
© 2006 Semtech Corp.
ON
)
9
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 en-
abled 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 SC488
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
To SC488 FB (pin 9)
is a resistor connected between the input supply and
3.3x10
12
To VDDQ output capacitor
(RTON
Figure 2
R2
37x10
R3
) 3
C
V
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V
OUT
IN
SC488
50
ns