sc483itstrt Semtech Corporation, sc483itstrt Datasheet - Page 8

• Manufacturer Part Number: sc483itstrt
• Description: Sc483 Dual Synchronous Buck Pseudo-fixed-frequency Power-supply Controller
• Manufacturer: Semtech Corporation

+5V Bias Supplies
The SC483 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 such as the Semtech LP2951.
To avoid interference between outputs, each controller
has its own ground reference, VSSA, which should be
tied by a single trace to PGND at the negative terminal of
that controller’s output capacitor (see Layout Guidelines).
All external components referenced to VSSA in the
schematic should be connected to the appropriate VSSA
trace. The supply decoupling capacitor for controller 1
should be tied between VCCA1 and VSSA1. Likewise, the
supply decoupling capacitor for controller 2 should be
tied between VCCA2 and VSSA2. A 10
be used to decouple each VCCA supply from the main
VDDP supplies. PGND can then be a separate plane which
is not used for routing traces. All PGND connections are
connected directly to the ground plane with special
attention given to avoiding indirect connections which
may create ground loops. As mentioned above, VSSA1
and VSSA2 must be connected to the PGND plane at
the negative terminal of their respective output
capacitors only. The VDDP1 and VDDP2 inputs provide
power to the upper and lower gate drivers. A decoupling
capacitor for each supply is required. No series resistor
between VDDP and 5V is required. See layout guidelines
for more details.
Pseudo-fixed Frequency Constant On-Time PWM
Controller
The PWM control architecture consists of a constant on-
time, pseudo fixed frequency PWM controller (see Figure
1, SC483 Block Diagram). The output ripple voltage
developed across the output filter capacitor’s ESR
provides the PWM ramp signal eliminating the need for a
current sense resistor. The high-side switch on-time is
determined by a one-shot whose period is directly
proportional to output voltage and inversely proportional
to input voltage. A second one-shot sets the minimum
off-time which is typically 400ns.
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 current.
POWER MANAGEMENT
Applications Information
2005 Semtech Corp.
ON
)
resistor should
8
This input voltage-proportional current is used to charge
an internal on-time capacitor. The on-time is the time
required for the voltage on 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 for a clock generator.
For VOUT < 3.3V:
For 3.3V
R
to the TON pin. Due to the high impedance of this
resistor, the TON pin should always be bypassed to VSSA
using a 1nF ceramic capacitor.
EN/PSV: Enable, Psave and Soft Discharge
The EN/PSV pin enables the supply. When EN/PSV is
tied to VCCA the controller is enabled and power save
will also be enabled. When the EN/PSV pin is tri-stated,
an internal pull-up will activate the controller and power
save will be disabled. If PSAVE is enabled, the SC483
PSAVE comparator will look for the inductor current to
cross zero on eight consecutive switching cycles by
comparing the phase node (LX) to PGND. Once observed,
the controller will enter power save and turn off the low
side MOSFET when the current crosses zero. To improve
light-load efficiency and add hysteresis, the on-time is
increased by 50% in power save. The efficiency
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 will immediately exit 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 psave is enabled.
If the EN/PSV pin is pulled low, the related output will be
shut down and discharged using a switch with a nominal
resistance of 22 Ohms. This will ensure that the output
is in a defined state next time it is enabled and also
t
t
ON
ON
TON
is a resistor connected from the input supply (VBAT)
3
. 0
3 .
85
x
10
VOUT
3
3 .
12
x
10
R (
5V:
TON
12
R (
37
TON
x
10
37
3
)
x
10
V
V
3
OUT
BAT
)
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V
V
SC483
OUT
BAT
50
ns
50
ns