LTC3122_15 Datasheet, PDF (9/26 Pages) Linear Technology – 15V, 2.5A Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3122
Operation
The LTC3122 is an adjustable frequency, 100kHz to 3MHz
synchronous boost converter housed in either a 12-lead
4mm × 3mm DFN or a thermally enhanced MSOP pack-
age. The LTC3122 offers the unique ability to start-up
and regulate the output from inputs as low as 1.8V and
continue to operate from inputs as low as 0.5V. Output
voltages can be programmed between 2.2V and 15V. The
device also features fixed frequency, current mode PWM
control for exceptional line and load regulation. The cur-
rent mode architecture with adaptive slope compensation
provides excellent transient load response and requires
minimal output filtering. An internal 10ms closed loop
soft-start simplifies the design process while minimizing
the number of external components.
With its low R_DS(ON) and low gate charge internal N-channel
MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3122 achieves high efficiency over a wide
range of load current. High efficiency is achieved at light
loads when Burst Mode operation is commanded. Operation
can be best understood by referring to the Block Diagram.
Low Voltage Operation
The LTC3122 is designed to allow start-up from input
voltages as low as 1.8V. When V_OUT exceeds 2.2V, the
LTC3122 continues to regulate its output, even when V_IN
falls to as low as 0.5V. The limiting factors for the applica-
tion become the availability of the input source to supply
sufficient power to the output at the low voltages, and
the maximum duty cycle. 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. This feature extends operating times by
maximizing the amount of energy that can be extracted
from the input source.
zero to its final programmed value. This limits the inrush
current drawn from the input source. As a result, the du-
ration 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 <
1.6V), an overvoltage event on V_OUT (V_OUT ≥ 16.2V), or
an overtemperature event (thermal shutdown is invoked
when the die temperature exceeds 170°C). Upon removal
of these fault conditions, the LTC3122 will soft-start the
output voltage.
Error Amplifier
The non-inverting input of the transconductance error
amplifier is internally connected to the 1.202V reference
and the inverting input is connected to FB. An external
resistive voltage divider from V_OUT to ground programs
the output voltage from 2.2V to 15V via FB as shown in
Figure 1.
V_OUT
=
1.202V ^{⎛⎝⎜}1+
R1⎞
R2 ⎠⎟
Selecting an R2 value of 121kΩ to have approximately
10µA of bias current in the V_OUT resistor divider yields
the formula:
R1 = 100.67•(V_OUT – 1.202V)
where R1 is in kΩ.
Power converter control loop compensation is set by a
simple RC network between V_C and ground.
LTC3122
V_OUT
Low Noise Fixed Frequency Operation
R1
FB
Soft-Start
1.202V
R2
The LTC3122 contains internal circuitry to provide closed-
loop soft-start operation. The soft-start utilizes a linearly
increasing ramp of the error amplifier reference voltage
from zero to its nominal value of 1.202V in approximately
10ms, with the internal control loop driving V_OUT from
3122 F01
Figure 1. Programming the Output Voltage
3122fa
For more information www.linear.com/LTC3122
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