LT8331 datasheet(9/34 Pages) AD

9 / 34 page

LT8357

Rev. 0

For more information www.analog.com

OPERATION

Main Control Loop

The LT8357 uses a fixed frequency, current mode con-

trol scheme to provide excellent line and load regulation.

Operation can be best understood by referring to the

Block Diagram. The inductor current is sensed through

the current sense resistor from the SENSE pin to ground.

The current sense voltage is gained up by amplifier A2

and added to a slope compensation ramp signal from the

internal oscillator. The summing signal is then fed into

the positive terminal of the current comparator A1. The

negative terminal of A1 is controlled by the voltage on the

VC pin, which is the output of the error amplifier EA. The

EA amplifies the difference between the feedback voltage

on the FB pin and the 1V reference voltage, and adjusts

the VC voltage accordingly to set the correct peak switch

current level to keep the output voltage in regulation.

Light Load Current Operation

At light load, the user can program the SYNC/MODE pin

to allow the LT8357 running either in the pulse-skipping

mode or low ripple Burst Mode operation. When operat-

ing in the pulse-skipping mode, the power switches are

held off for multiple clock cycles (i.e., skipping pulses)

to maintain the regulation and improve the efficiency. To

further enhance the efficiency while minimizing the input

quiescent current and output voltage ripple, the LT8357

can also operate in the low ripple Burst Mode. When in the

Burst Mode operation, the LT8357 delivers a single small

pulse of current to the output capacitor every switching

cycle, followed by a long sleep period where the output

power is supplied by the output capacitor. During the

sleep period, the LT8357 shuts down most of the cir-

cuits and thus consumes very low (8μA typical) input

quiescent current.

As the output load decreases, the frequency of the single

current pulse decreases (see Figure 1) and the percent-

age of sleep period increases, resulting in much higher

light load efficiency than typical converters. To optimize

the quiescent current performance at light load, the cur-

rent in the feedback resistor divider must be minimized

as it appears to the output as load current. In addition, all

possible leakage currents from the output should also be

minimized as they all add to the equivalent output load.

The largest contributor to leakage current can be due to

the reverse biased leakage of the output diode (see the

Output Diode Selection in the Applications Information

section).

FRONT PAGE APPLICATION

VIN = 12V, VOUT = 24V

LOAD CURRENT (A)

0.0

0.4

0.8

1.2

1.6

0.5

1.0

1.5

2.0

2.5

8357 F01

Figure 1. Burst Frequency vs Load Current

When in Burst Mode operation, the switch current limit is

approximately 10mV (typical) on the SENSE pin, resulting

in low output voltage ripple. Increasing the output capaci-

tance will decrease the output ripple proportionally. As

the output load ramps upward from zero, the switching

frequency will increase but only up to the fixed oscillator

frequency set by the resistor on the RT pin, as shown in

Figure 1. The output load at which the LT8357 reaches

the fixed frequency varies based on input voltage, output

voltage and inductor choice.

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