SC411EVB SEMTECH [Semtech Corporation], SC411EVB Datasheet - Page 9

• Manufacturer Part Number: SC411EVB
• Description: Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller
• Manufacturer: SEMTECH [Semtech Corporation]

+5V Bias Supplies
The SC411 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 such as the Semtech LP2951.
For optimal operation, the controller has its own ground
reference, VSSA, which should be tied along with PGND
directly to the thermal pad under the part, which in turn
should connect to the ground plane using multiple vias.
All external components referenced to VSSA in the Typical
Application Circuit on Page 1 located near their respec-
tive pins. Supply decoupling capacitors should be located
adjacent to their respective pins. A 10Ω resistor should
be used to decouple VCCA from the main VDDP supply. All
ground connections are connected directly to the ground
plane as mentioned above. VSSA and PGND should be
starred at the thermal pad. 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 on page 17 for
more details.
Pseudo-Fixed Frequency Constant On-Time PWM
Controller
The PWM control architecture consists of a constant on-
time, pseudo fi xed frequency PWM controller (Block Dia-
gram, Page 6). The output ripple voltage developed across
the output fi lter 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 re-
quired 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
© 2007 Semtech Corp.
POWER MANAGEMENT
Application Information
ON
)
9
in a nearly constant switching frequency without the need
for a clock generator.
For VOUT < 3.3V:
For 3.3V ≤ VOUT ≤ 5V:
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 SC411 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 ef-
fi ciency and add hysteresis, the on-time is increased by
50% in power save. The effi ciency improvement at light-
loads more than offsets the disadvantage of slightly high-
er 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 cross-
ings, the converter can sink current as long as the cur-
rent 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 defi ned state next time it is enabled and also ensure,
since this is a soft discharge, that there are no danger-
ous negative voltage excursions to be concerned about. In
order for the soft discharge circuitry to function correctly,
the chip supply must be present.
TON
t
is a resistor connected from the input supply (VBAT)
ON
t
ON
=
=
. 0
3
85
3 .
x
•
10
3
3 .
−
12
x
10
•
R (
−
12
T ON
•
R (
+
37
T ON
x
+
10
37
3
)
x
•
10
⎛
⎜ ⎜
⎝
V
V
3
)
OUT
BAT
•
⎛
⎜ ⎜
⎝
⎞
⎟ ⎟
⎠
V
V
www.semtech.com
+
OUT
BAT
50
SC411
⎞
⎟ ⎟
⎠
ns
+
50
ns