LT8331 datasheet(16/24 Pages) AD

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LT8337/LT8337-1

Rev. 0

For more information www.analog.com

Figure 3. A Simplified LT8337/LT8337-1 Power

Stage with a Diode Added Between VIN and VOUT

switcher from attaining a sufficiently low duty cycle. The

typical switching waveforms in these VIN approaching

VOUT conditions are shown in the Typical Performance

Characteristics section.

Start-Up

To limit the peak switch current and VOUT overshoot dur-

ing start-up, the LT8337/LT8337-1 contains internal cir-

cuitry to provide soft-start operation (refer to the error

amplifier EA in Block Diagram). During start-up, the inter-

nal soft-start circuity slowly ramps the internal SS signal

from zero to 1V. When the SS voltage falls between the FB

initial voltage and 1V, the IC regulates the FB pin voltage

to the SS voltage instead of 1V. In this way the output

capacitor is charged gradually towards its final value while

limiting the start-up peak switch currents.

Referring to Figure 2, the start-up time TSTART_UP is the

time period from EN/UVLO transitioning high to VOUT hav-

ing reached 90% of its regulation voltage programmed

by FB resistor network. When VIN > 3.6V, TSTART_UP is

approximately given by Equation 7.

TSTART_UP = 0.25ms +

2100

fSW

(7)

The IC selects pulse-skipping mode with no spread

spectrum frequency modulation during start-up, and the

SYNC/MODE pin configuration is ignored. The IC reads

SYNC/MODE pin configuration after the start-up delay

(Equation 9).

TMODE_DELAY = 0.22ms +

4096

fSW

(9)

If the LT8337/LT8337-1 boost converter is plugged into

a live supply, the VOUT could ring to twice the voltage of

VIN, due to the resonant circuit composed by L, COUT1-3,

and the body diode of M2 (refer to Block Diagram). If such

over-shoot exceeds the VOUT rating, it must be limited

to protect the load and the converter. For these situa-

tions, a small Schottky diode or silicon diode can be con-

nected between VIN and VOUT to deactivate the resonant

circuit and limit the VOUT over-shoot as shown in Figure 3.

With the diode connected, the boost is also more robust

against output fault conditions such as output short cir-

cuit or overload, due to the fact that the diode diverts a

great amount of output current from the IC. The diode

can be rated for about one half to one fifth of the full load

current since it only conducts current during start-up or

output fault conditions.

APPLICATIONS INFORMATION

VIN

GND

VOUT

CIN

COUT1,2

COUT3

LT8337/LT8337-1

83371 F03

Figure 2. Typical Start-Up Waveforms

1ms/DIV

2A/DIV

VOUT

5V/DIV

EN/UVLO

2V/DIV

VIN = 7.2V

FRONT PAGE CIRCUIT

15V

83371 F02

tMODE_DELAY

tSTART_UP

Inductor Selection

When operating in continuous conduction mode (CCM),

the duty cycle can be calculated based on the output volt-

age (VOUT) and the input voltage (VIN). The maximum duty

cycle (DMAX) occurs when the converter has the minimum

input voltage (Equation 10).

DMAX =

VOUT – VIN(MIN)

VOUT

(10)

When VIN < 3.6V, TSTART_UP is approximately given by

Equation 8.

TSTART_UP = 0.25ms +

3.5V

VIN − 0.1V

•

2100

fSW

(8)

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