LTC3124_15 Datasheet, PDF (11/28 Pages) Linear Technology – 15V, 5A 2-Phase Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3124
OPERATION
SWITCH A
VOLTAGE
SWITCH B
VOLTAGE
INDUCTOR A
CURRENT
INDUCTOR B
CURRENT
INPUT
CURRENT
RECTIFIER A
CURRENT
RECTIFIER B
CURRENT
OUTPUT
RIPPLE
CURRENT
3124 F02
Figure 2. Simplified Voltage and Current Waveforms
for 2-Phase Operation at 50% Duty Cycle
SWITCH A
VOLTAGE
SWITCH B
VOLTAGE
INDUCTOR A
CURRENT
INDUCTOR B
CURRENT
INPUT
CURRENT
RECTIFIER A
CURRENT
LOW VOLTAGE OPERATION
The LTC3124 is designed to allow start-up from input
voltages as low as 1.8V. When V_OUT exceeds 2.5V, the
LTC3124 continues to regulate its output, even when V_IN
falls as low as 0.5V. This feature extends operating times
by maximizing the amount of energy that can be extracted
from the input source. The limiting factors for the applica-
tion become the availability of the power source to supply
sufficient power to the output at the low input voltage,
and the maximum duty cycle, which is clamped at 94%.
Note that at low input voltages, small voltage drops due
to series resistance become critical and greatly limit the
power delivery capability of the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The LTC3124 contains internal circuitry to provide soft-
start operation. The soft-start utilizes a linearly increasing
ramp of the error amplifier reference voltage from zero
to its nominal value of 1.2V in approximately 10ms, with
the internal control loop driving V_OUT from zero to its
final programmed value. This limits the inrush current
drawn from the input source. As a result, the duration
of the soft-start is largely unaffected by the size of the
output capacitor or the output regulation voltage. The
closed-loop nature of the soft-start allows the converter
to respond to load transients that might occur during
the soft-start interval. The soft-start period is reset by a
shutdown command on SD, a UVLO event on V_CC (V_CC <
1.5V), an overvoltage event on V_OUT (V_OUT ≥ 16.5V), or
an overtemperature event (TSD is invoked when the die
temperature exceeds 170°C). Upon removal of these fault
conditions, the LTC3124 will soft-start the output voltage.
RECTIFIER B
CURRENT
Error Amplifier
OUTPUT
RIPPLE
CURRENT
3124 F03
Figure 3. Simplified Voltage and Current Waveforms
for 2-Phase Operation at 25% Duty Cycle
The noninverting input of the transconductance error
amplifier is internally connected to the 1.2V reference and
the inverting input is connected to FB. An external resistive
voltage divider from V_OUT to SGND programs the output
voltage from 2.5V to 15V via FB as shown in Figure 4.
V_OUT
=
1.2V


1+
R1
R2 ^
3124f
For more information www.linear.com/LTC3124
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