LTC3458L

10

3458lf

transition occurs using a single resistor. During Burst

Mode operation, the oscillator is shut down, 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 deter-

mined 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

voltage has a variable frequency component with load

current and will be typically 2% peak-to-peak. This maxi-

mizes efficiency at very light loads by minimizing switch-

ing and quiescent losses. Burst Mode ripple can be re-

duced slightly by using more output capacitance (22

µF or

greater). This capacitor does not need to be a low ESR type

if low ESR ceramics are also used. Another method of

reducing Burst Mode ripple is to place a small feed-

feedback divider network.

During Burst Mode operation, COMP is disconnected

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

the minimum and maximum voltage on COMP during

Burst Mode operation. This minimizes the transient expe-

rienced 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 the BURST pin 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

=

10

C

CV

BURST

OUT

OUT

=

•

,

10 000

Note: the BURST pin only sources current based on

If current in the inductor is allowed to go negative (this can

occur at very light loads and high step-up ratios), the burst

threshold may become inaccurate, preventing the IC from

than 200k, a larger than recommended inductor value may

be needed to ensure positive inductor current and auto-

matic Burst Mode operation.

In the event that a sudden load transient causes the voltage

level on FB to drop 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

transient response when going between Burst Mode op-

eration and PWM mode, Burst can be controlled manually

by the host. This way PWM mode can be commanded

before the load step occurs, minimizing output voltage

drop. Note that Burst Mode operation is inhibited during

start-up and soft-start.

Manual Control

For applications requiring fixed frequency operation at all

51k resistor. To force Burst Mode operation, ground the

BURST pin.

For applications where a large load step can be anticipated,

the circuit below can be used to reduce the voltage transient

nal P-channel MOSFET is off and fixed frequency operation

is commanded when the external P-channel MOSFET is on.

In shutdown, the P-channel MOSFET should be off.

APPLICATIO S I FOR ATIO

Figure 1

0.01

µF

BURST

133k

P-CHANNEL

MOSFET

HIGH: AUTO MODE

LOW: FIXED FREQUENCY

3458L FO1