LTC3422

9

3422fa

OPERATIO

radiation.

Burst Mode OPERATION

Burst Mode operation can be automatic or user controlled.

In automatic operation, the LTC3422 will automatically

enter Burst Mode operation at light load and return to fixed

frequency PWM mode for heavier loads. The user can

program the average load current at which the mode

transition occurs using a single resistor connected from

BURST to GND.

The oscillator is shut down during Burst Mode operation,

since the on time is determined by the time it takes the

inductor current to reach a fixed 600mA peak current and

the off time is determined by the time it takes for the

inductor current to return to zero.

In Burst Mode operation, the LTC3422 delivers energy to

the output until it is regulated and then enters a sleep state,

where the switches are kept off while the LTC3422 con-

sumes only 25

µA of quiescent current. In this mode the

output ripple has a variable frequency component with

load current and will be typically 2% peak-peak. This

maximizes efficiency at very light loads by minimizing

switching and quiescent losses. Burst Mode operation

ripple can be reduced slightly by increasing the output

capacitance (47

µF or greater). This additional capacitance

does not need to be a low ESR type if low ESR ceramics are

also used. Another method of reducing Burst Mode opera-

tion ripple is to place a small feed-forward capacitor (10pF

divider network.

In Burst Mode operation, the compensation network is not

During long periods of Burst Mode operation, leakage

currents in the external components or on the PC board

could cause the compensation capacitor to charge (or

discharge), which could result in a large output transient

when returning to fixed frequency mode of operation, even

at the same load current. To prevent this, the LTC3422

incorporates an active clamp circuit that holds the voltage

This minimizes any output transient when returning to

fixed frequency mode operation.

Automatic Burst Mode Operation Control

For automatic operation, an RC network should be con-

nected from BURST to ground. The value of the resistor

Burst Mode operation will be entered and exited (there is

hysteresis to prevent oscillation between modes). The

equation given for the capacitor on BURST is the minimum

value to prevent ripple on BURST from causing the part to

oscillate in and out of Burst Mode operation at the current

where the mode transition occurs. The equation given for

the resistor on BURST is the typical average load current

at which automatic Burst Mode operation is exited.

R

I

B

EXITBURST

=

12

C

CV

B

OUT

OUT

≥

•

,

64 000

Please refer to the Burst Mode Output Current Threshold

In the event that a load transient causes FB to drop by more

than 4% from the regulation value while in Burst Mode

operation, the LTC3422 will immediately switch to fixed

frequency operation and an internal pull-up will be mo-

mentarily applied to BURST, rapidly charging the BURST

capacitor. This prevents the LTC3422 from immediately

re-entering Burst Mode operation once the output achieves

regulation.

Manual Burst Mode Operation

For optimum transient response with large dynamic loads,

the operating mode should be controlled manually by the

host. By commanding fixed frequency PWM operation

prior to a sudden increase in load, output voltage droop

can be minimized. For manual control of Burst Mode

operation, the RC network connected to BURST can be

eliminated. To force fixed frequency PWM mode, BURST

tion, BURST should be grounded. When commanding

Burst Mode operation manually, the circuit connected to