LT8331 datasheet(11/24 Pages) AD

11 / 24 page

LT8337/LT8337-1

Rev. 0

For more information www.analog.com

OPERATION

The LT8337/LT8337-1 uses a fixed frequency, current

mode control scheme to provide excellent line and load

regulation. Referring to the Block Diagram, the Switching

Logic and Charge Pump block turns on the power switch

M1 through driver G1 at the start of each oscillator cycle.

During the M1 switch on-phase, the inductor current IL

flows through M1. A current proportional to the M1 switch

current is added to a stabilizing slope compensation ramp

and the resulting sum is fed into the positive terminal

of the PWM comparator A1. The voltage at the negative

input of A1, labeled “VC”, is set by the error amplifier EA

and is an amplified version of the difference between the

feedback voltage FB and the reference voltage. During

the M1 on-phase, IL increases. When the signal at the

positive input of A1 exceeds VC, A1 sends out a signal to

the Switching Logic and Charge Pump block to turn off

M1. When M1 turns off, the synchronous power switch

M2 turns on until the next clock cycle begins or induc-

tor current IL falls to zero. During the M1 off-phase, IL

decreases. Through this repetitive action, the EA sets the

correct IL peak current level to keep the output in regula-

tion. VIN and VOUT are constantly monitored by the IC.

When VIN rises above VOUT (causing A3’s output high)

and at the same time VOUT is higher than its regulation

voltage programmed by the FB resistor network, the IC

enters PassThru operation, where M2 is kept on continu-

ously and M1 is kept off continuously, and the VOUT is

essentially shorted to VIN by the inductor and M2. See

Applications Information section for further details.

The IC features Silent Switcher architecture to minimize

EMI emissions while delivering high efficiency. The Silent

Switcher EMI cancellation loops are completed by placing

one 1µF capacitor between VOUT at pin 6 and GND at pin 5

and a matching 1µF capacitor between VOUT at pin 7 and

GND at pin 8 (see Applications Information section for

further details).

The EN/UVLO pin controls whether the IC is enabled or

is in shutdown state. A 1.0V reference and a comparator

A2 with 90mV hysteresis (Block Diagram) allow the user

to accurately program the supply voltage at which the IC

turns on and off. See the Applications Information section

for further details.

The LT8337/LT8337-1 features a variety of operation

modes which can be selected by SYNC/MODE pin to opti-

mize the converter performance based on the application

requirements. The low ripple Burst Mode operation can

be selected to optimize the efficiency at light loads. The

spread spectrum frequency modulation function can be

selected to minimize the EMI emissions.

Pulling SYNC/MODE pin to ground selects Burst Mode

operation. Connecting this SYNC/MODE to ground

through a 50k resistor selects Burst Mode operation with

spread spectrum frequency modulation. Floating SYNC/

MODE pin selects pulse-skipping operation. Connecting

SYNC/MODE pin to INTVCC selects pulse-skipping oper-

ation with spread spectrum frequency modulation. If a

clock is applied to the SYNC/MODE pin, the IC synchro-

nizes to an external clock frequency and operates in pulse-

skipping mode. See the Applications Information section

for further details.

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