sc1470 Semtech Corporation, sc1470 Datasheet - Page 7

• Manufacturer Part Number: sc1470
• Description: Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

+5V Bias Supplies
The SC1470 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.
For optimal operation, the controller has its own ground
reference, VSSA, which should be tied by a single trace
to PGND at the negative terminal of the output
capacitor (see Layout Guidelines). All external components
referenced to VSSA in the Typical Applications Circuit on
Page 1 should be connected to VSSA. The supply
decoupling capacitor should be tied directly between the
VCCA and VSSA pins. A 10
decouple VCCA from the main VDDP supply. 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, VSSA must be connected to the PGND
plane at the negative terminal of the output capacitor
only. The VDDP input provides power to the upper and
lower gate drivers. A decoupling capacitor 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, SC1470 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.
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
POWER MANAGEMENT
Application Information
2005 Semtech Corp.
ON
)
resistor should be used to
7
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.
Enable & Psave
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. If PSAVE is enabled, the SC1470 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.
When the EN/PSV pin is tri-stated, an internal pull-up
will activate the controller and power save will be
disabled.
When the EN/PSV pin is pulled low, the supply is disabled
and the MOSFET drivers are tri-stated.
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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SC1470
V
V
OUT
BAT
50
ns
50
ns