SC2449 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2449' PDF : 20 Pages View PDF
POWER MANAGEMENT
Electrical Characteristic Curves (Cont.)
Start-up (Vin = 24V, Vout = 3.3V, Load Current = 5A/phase)
SC2449
ch1: Vout
Theory of Operation
The SC2449 employs a voltage mode control with feed
forward to provide fast output response to load and line
transients.
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%.
The SC2449 has two outputs, which can be used to gen-
erate two separate supply voltages or can be combined Oscillator .requency Selection
in bi-phase operation to generate one single supply volt-
age. The internal reference is trimmed to 1 V with +/-1% The sawtooth signal is generated by charging an internal
accuracy, and the outputs voltages can be adjusted by capacitor with a current source. The charge current is
two external resistors. In bi-phase operation, the dual set by an external resistor connected from the RRE. pin
switching regulators are operated 180° out of phase. to AGND. The oscillator frequency and the external resis-
Load current sharing between phases is normally re- tance follow an inversely proportional relationship.
quired, and this can be achieved by using precise feed-
back voltage divider resistors (typically 0.1%) to match .eed .orward
individual phase output voltage. In addition, small droop-
ing resistors ( could be PCB traces) are employed at the The SC2449 incorporates a voltage feed forward scheme
output of each phase to enhance phase current balance. to improve line transient immunity when changes of the
input voltage occur. As the input voltage changes, the
PWM Control
ramp valley to peak voltage of the internal oscillator fol-
lows this change instantly. As a result the output voltage
Changes on the output voltages are fed to the inverting will have minimum disturbance due to the input line
input of the Error Amplifiers, by the .B1 and .B2 pins, change.
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 Synchronized Operation
the COMP1 and COMP2 pins to AGND respectively. The
error signal from the error amplifier is compared to the The internal oscillator can be synchronized to an exter-
saw tooth waveform by the PWM comparator, and nal clock operating in the range of 270 kHz to 1 MHz.
2002 Semtech Corp.
14
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