sc2449iswtrt Semtech Corporation, sc2449iswtrt Datasheet - Page 14

• Manufacturer Part Number: sc2449iswtrt
• Description: Sc2449 Bi-phase/dual Controller
• Manufacturer: Semtech Corporation

Start-up (Vin = 24V, Vout = 3.3V, Load Current = 5A/phase)
The SC2449 employs a voltage mode control with feed
forward to provide fast output response to load and line
transients.
The SC2449 has two outputs, which can be used to gen-
erate two separate supply voltages or can be combined
in bi-phase operation to generate one single supply volt-
age. The internal reference is trimmed to 1 V with +/-1%
accuracy, and the outputs voltages can be adjusted by
two external resistors. In bi-phase operation, the dual
switching regulators are operated 180° out of phase.
Load current sharing between phases is normally re-
quired, and this can be achieved by using precise feed-
back voltage divider resistors (typically 0.1%) to match
individual phase output voltage. In addition, small droop-
ing resistors ( could be PCB traces) are employed at the
output of each phase to enhance phase current balance.
PWM Control
Changes on the output voltages are fed to the inverting
input of the Error Amplifiers, by the FB1 and FB2 pins,
and compared with the internal 1 V reference. The com-
pensation to the transconductance amplifier is achieved
by connecting a capacitor in series with a resistor from
the COMP1 and COMP2 pins to AGND respectively. The
error signal from the error amplifier is compared to the
saw tooth waveform by the PWM comparator, and
Theory of Operation
Electrical Characteristic Curves (Cont.)
POWER MANAGEMENT
2007 Semtech Corp.
ch1: Vout
14
matched timing signal is generated to control the upper
and lower gate drives of the two phases. A single Ramp
signal is used to generate the control signals for both of
phases, hence the maximum duty cycle is less than 50%.
Oscillator Frequency Selection
The sawtooth signal is generated by charging an internal
capacitor with a current source. The charge current is
set by an external resistor connected from the RREF pin
to AGND. The oscillator frequency and the external resis-
tance follow an inversely proportional relationship.
Feed Forward
The SC2449 incorporates a voltage feed forward scheme
to improve line transient immunity when changes of the
input voltage occur. As the input voltage changes, the
ramp valley to peak voltage of the internal oscillator fol-
lows this change instantly. As a result the output voltage
will have minimum disturbance due to the input line
change.
Synchronized Operation
The internal oscillator can be synchronized to an exter-
nal clock operating in the range of 270 kHz to 1 MHz.
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SC2449