LTC3421

9

3421f

OPERATIO

Zero Current Amplifier

The zero current amplifier monitors the inductor current to

the output and shuts off the synchronous rectifier once the

current is below 50mA typical, preventing negative induc-

tor current.

Antiringing Control

The antiringing control places a resistor across the

inductor to damp the ringing on the SW pin in discontinu-

cause EMI radiation.

The internal 1.22V reference is buffered and brought out

to REFOUT. It is active when the ENB pin is pulled high

(above 1.4V). For stability, a minimum of a 0.1

µFcapacitor

must be placed on the pin. The output can source up to

100

µA and sink up to 8µA. For the lowest possible quies-

cent current in Burst Mode operation, the reference output

should be disabled by grounding the ENB pin.

Burst Mode OPERATION

Burst Mode operation can be automatic or user controlled.

In automatic operation, the IC 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.

The oscillator is shut down in this mode, since the on time

is determined by the time it takes the inductor current to

reach a fixed 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 IC delivers energy to the

output until it is regulated and then goes into a sleep mode

where the outputs are off and the IC is consuming only

12

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 ripple can be reduced slightly by

increasing the output capacitance. Another method of

reducing Burst Mode ripple is to place a small feed-

feedback divider network.

from the error amplifier in an effort to hold the voltage on

the external compensation network where it was before

entering Burst Mode operation. To minimize the effects of

leakage current and stray resistance, voltage clamps limit

tion. This minimizes the transient experienced when a

heavy load is suddenly applied to the converter after being

in Burst Mode operation for an extended period of time.

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 for 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.

R

I

BURST

BURST

=

2

C

CV

BURST

OUT

OUT

≥

•

,

10 000

In the event that a sudden load transient causes FB to

deviate by more than 4% from the regulation value, an

internal pull-up is applied to BURST, forcing the part

quickly out of Burst Mode operation. For optimum tran-

sient response when going between Burst Mode operation

and PWM mode, the mode should be controlled manually

by the host. This way PWM mode can be commanded

before the load step occurs, minimizing output voltage

droop. For manual control of Burst Mode operation, the

RC network can be eliminated. To force fixed frequency

force Burst Mode operation, BURST should be grounded.