SC415 Datasheet, PDF (11/30 Pages) Semtech Corporation – Dual Synchronous Buck Controller

SC415
Applications Information (continued)
The voltages available are shown:
FB Internal Voltage Selection
Side1
Side2
FB = RTN
1.5V
1.05V
FB = +5V
1.8V
1.25V
Enable/Psave Inputs
Each converter has a separate Enable pin. Each EN input
operates as follows:
EN = GND. This turns the converter off.
EN = open (float). This turns the converter on with psave
mode disabled (continuous conduction mode). In this
case, the EN pin will float to approximately 2V due to
an internal 1.5Meg/1Meg resistor divider from the +5V
supply at VDDx to ground.
EN = high (3.1V min). This turns the converter on with
psave mode enabled. At light loads, the converter will
operate in psave mode.
Note that the two EN pins are separate, so each output
can be disabled or operated with or without psave
independently.
If both EN1 and EN2 are grounded, the device is placed
into the lowest-power state, drawing typically 10μA from
the +5V supply.
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 immediately exits PSAVE. Once PSAVE
is exited, it requires 8 switching cycles at light load to re-
enter PSAVE. 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.
When operating in PSAVE mode at light loads, the LX
waveform will not have the typical square wave shape
seen when operating in continuous conduction mode.
Shortly after DL drives low, and both MOSFETs are off, the
LX voltage will show ringing. This ringing is caused by the
LC circuit formed by the inductor and device capacitance
of the MOSFETs and low-side diode. When the low-
side MOSFET turns off the inductor current falls toward
zero. When it reaches zero, the inductance and MOSFET
capacitances will tend to ring freely. This is normal PSAVE
operation as shown in Figure 3:
VIN
DH Q1
CQ1
L
VOUT
DL Q2
COUT
CQ2
PSAVE Operation
Each output provides automatic PSAVE operation at
light loads if the ENx pin is set high. The internal Zero-
Cross comparator looks for inductor current (via the
voltage across the lower MOSFET) to fall to zero on
eight consecutive switching cycles. Once observed,
the controller enters PSAVE mode and turns off the
low-side MOSFET on each subsequent cycle when the
current crosses zero. To add hysteresis, the on-time is also
increased by 25% when PSAVE mode is active, for that
output only; it does not affect the other converter. The
DL
VIN
VOUT
LX
Typical ringing at LX
Figure 3
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