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LTC3121

3121fa

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operaTion

The LTC3121 is an adjustable frequency, 100kHz to 3MHz

synchronous boost converter housed in a 12-lead 4mm ×

3mm DFN. The LTC3121 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.

WithitslowRDS(ON)andlowgatechargeinternalN-channel

MOSFET switch and P-channel MOSFET synchronous

rectifier, the LTC3121 achieves high efficiency over a wide

range of load current. High efficiency is achieved at light

loadswhenBurstModeoperationiscommanded.Operation

can be best understood by referring to the Block Diagram.

LOW VOLTAGE OPERATION

The LTC3121 is designed to allow start-up from input

voltages as low as 1.8V. When VOUT exceeds 2.2V, the

LTC3121 continues to regulate its output, even when VIN

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.

LOW NOISE FIXED FREQUENCY OPERATION

Soft-Start

The LTC3121 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 VOUT from

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 VCC (VCC <

1.6V), an overvoltage event on VOUT (VOUT ≥ 16.2V), or

an overtemperature event (thermal shutdown is invoked

when the die temperature exceeds 170°C). Upon removal

of these fault conditions, the LTC3121 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 VOUT to ground programs

the output voltage from 2.2V to 15V via FB as shown in

Figure 1.

VOUT = 1.202V 1+

⎛

⎝

⎜

⎞

⎠

⎟

Selecting an R2 value of 121kΩ to have approximately

10µA of bias current in the VOUT resistor divider yields

the formula:

R1 = 100.67•(VOUT – 1.202V)

where R1 is in kΩ.

Power converter control loop compensation is set by a

simple RC network between VC and ground.

Figure 1. Programming the Output Voltage

3121 F01

LTC3121

1.202V

VOUT

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