8
© 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4501
Operation
The SC4501 is a programmable constant-frequency peak
current-mode step-up switching regulator with an
integrated 2A power transistor. Referring to the block
diagrams in Figures 2 and 3, the power transistor is
switched on at the trailing edge of the clock. Switch
current is sensed with an integrated sense resistor. The
sensed current is summed with the slope-compensating
ramp before compared to the output of the error
amplifier EA. The PWM comparator trip point determines
the switch turn-on pulse width. The current-limit
comparator ILIM turns off the power switch when the
switch current exceeds the 2.8A current-limit threshold.
ILIM therefore provides cycle-by-cycle current limit.
Current-limit is not affected by slope compensation
because the current comparator ILIM is not in the PWM
signal path.
Current-mode switching regulators utilize a dual-loop
feedback control system. In the SC4501 the amplifier
output COMP controls the peak inductor current. This is
the inner current loop. The double reactive poles of the
output LC filter are reduced to a single real pole by the
inner current loop, easing loop compensation. Fast
transient response can be obtained with a simple Type-2
compensation network. In the outer loop, the error
amplifier regulates the output voltage.
The switching frequency of the SC4501 can be programmed
up to 2MHz with an external resistor from the ROSC pin
to the ground. For converters requiring extreme duty
cycles, the operating frequency can be lowered to
maintain the necessary minimum on time or the minimum
off time.
The SC4501 requires a minimum input of 1.4V to operate.
A voltage higher than 1.1V at the shutdown pin enables
the internal linear regulator REG in the SC4501. After V
REG
becomes valid, the soft-start capacitor is charged with a
1.5μA current source. A PNP transistor clamps the output
of the error amplifier as the soft-start capacitor voltage
rises. Since the COMP voltage controls the peak inductor
current, the inductor current is ramped gradually during
soft-start, preventing high input start-up current. Under
fault conditions (V
IN
<1.4V or over temperature) or when
the shutdown pin is pulled below 1.1V, the soft-start
capacitor is discharged to ground. Pulling the shutdown
pin below 0.1V reduces the total supply current to 10μA.
Setting the Output Voltage
An external resistive divider R
1
and R
2
with its center tap
tied to the FB pin (Figure 4) sets the output voltage.
⎟
⎠
⎞
⎜
⎝
⎛
−= 1
V242.1
V
RR
OUT
21
(1)
VOUT
R2
R1
40nA
FB
2
SC4501
The input bias current of the error amplifier will introduce
an error of:
()
%
V242.1
V
21
OUT
OUT
=
Δ
(2)
The percentage error of a V
OUT
= 5V converter with R
1
=
100KΩ and R
2
= 301KΩ is
()
%24.0
V242.1
V
OUT
OUT
==
Δ
Operating Frequency and Efficiency
Switching frequency of SC4501 is set with an external
resistor from the ROSC pin to the ground. A graph showing
the relationship between R
OSC
and switching frequency is
given in the “Typical Characteristics”.
High frequency operation reduces the size of passive
components but switching losses are higher. The efficiencies
of 5V to 12V converters operating at 700KHz, 1.35MHz
and 2MHz are shown in Figure 1(b). The peak efficiency
of the SC4501 appears to decrease 0.5% for every
100KHz increase in frequency.
Application Information
Figure 4. The Output Voltage is set with a Resistive Divider
