SC1401 Semtech Corporation, SC1401 Datasheet - Page 14

• Manufacturer Part Number: SC1401
• Description: HIGH PERFORMANCE SYNCHRONOUS BUCK CONTROLLER WITH LDO FOR PORTABLE POWER
• Manufacturer: Semtech Corporation

November 21, 2000
© 2000 SEMTECH CORP.
and output voltage and current ranges. This
compensation circuit is necessary for proper
operation of the SC1401. This will eliminate any
uncertainties in stability with respect to various output
configurations that may be implemented using the
part.
Having JP1 in place will enable the linear regulator to
derive power from the main switching supply. The
switcher must have a higher output voltage than the
linear to allow this option to work properly. Further
information of the evaluation board and it operation is
described in the SC1401 Evaluation Board Manual.
LDO
The evaluation board schematic shows that the LDO
regulator is set up to derive its power from the output
of the SMPS. The GATEIO, pin 16 is the gate drive
for an N-Channel MOSFET, Q1. This output has a
breakdown voltage of 12V so it is imperative the gate
does not exceed 12V and Q5, a nine volt zener diode
is added to prevent this from happening. In addition,
IOS, pin 17 is the input from the resistive divider
formed by R5 and R6. The LDO MOSFET should be
chosen to carry the load current and dissipate the
power required by the conditions of the load current
and voltage drop across the MOSFET. Capacitors
C9 and C10 were added for filtering purposes.
Gate Drive Power
JP4 enables the gate drive power to operate off of
the onboard zener follower which supplies 5 volts,
formed by components Q2, R15 and D4. This takes
the input voltage and produces 5V for the bootstrap
diode D1 that supplies the gate drive power for the
upper MOSFET of the Q3 SMPS. Alternatively, you
could also bring in an external voltage for the gate
drive voltage by moving the jumper from JP4 to JP3.
Consult the SC1401 Evaluation Board Manual for
further details of the evaluation board operation.
APPLICATIONS INFORMATION
Introduction
The SC1401 is a versatile switching regulator provid-
ing an adjustable output from 1.25V to 5.5V. In
addition, there is one on-chip adjustable linear
regulator capable of supplying 1 amp of output
current. The SC1401 is designed for notebook
applications but has applications anywhere high
HIGH PERFORMANCE SYNCHRONOUS BUCK
CONTROLLER WITH LDO FOR PORTABLE
POWER
L1
∆I
efficiency, small size and low cost are required.
The Semtech SC1401 EVAL board consists of a
2.0V, 7A switcher and a 1.5V, 1A linear regulator.
Design Guidelines
The schematic for the EVAL board is shown on page
10. The EVAL board is configured as follows:
Switching Regulator 1
Linear Regulator 1
Designing the Output Filter
Before calculating the output filter inductance and
output capacitor, an acceptable amount of output
ripple current is to be determined. The ESR of the
output capacitor multiplied by the ripple current sets
the maximum allowable ripple voltage. So once the
ripple voltage specification is selected, the capacitor
ESR is chosen usually based on capacitor cost and
size constraints. This then sets the maximum output
ripple current through the capacitor and inductor.
For the EVAL board 2.0V switcher, we selected a
maximum ripple voltage of 50mV. Choosing three
330uF, 6.3V tantalum capacitors C13,C14 and C15
each having an ESR of 100 milliohms, their
combined ESR equaling 33 milliohms, sets the
maximum ripple current as follows:
Be sure that the three output capacitors can handle
this ripple current. Ripple current specifications are
found in the capacitor data sheet for high quality
capacitors intended for use in switching power
supplies.
The inductance can now be found:
The inductance and duty cycle equation excludes
652 MITCHELL ROAD
O
(V
ESR
∆V
INMAX
O
∆I
V
O
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∆I
O
D
NEWBURY PARK CA 91320
0.033
t
0.05
Vout1 = 2.0V, 7A
Vout2 = 1.5V, 1A
1.5Amp
SC1401
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