5

LTC1626

OPERATIO

The nominal off-time of the LTC1626 is set by an external

The operating frequency is then determined by the off-

The output voltage is set by an external divider returned to

compare the divided output voltage with a reference

voltage of 1.25V.

To optimize efficiency, the LTC1626 automatically switches

between continuous and Burst Mode operation. The volt-

age comparator is the primary control element when the

device is in Burst Mode operation, while the gain block

controls the output voltage in continuous mode.

When the load is heavy, the LTC1626 is in continuous

operation. During the switch “ON” time, current compara-

with the inductor. When the voltage across the shunt

reaches the comparator’s threshold value, its output sig-

nal changes state, resetting the flip-flop and turning the

internal P-channel MOSFET off. The timing capacitor

rate determined by the off-time controller.

When the voltage on the timing capacitor has discharged

the switch to turn on. Also, the timing capacitor is

recharged. The inductor current will again ramp up until

the current comparator C trips. The cycle then repeats.

When the load current increases, the output voltage

decreases slightly. This causes the output of the gain stage

(Pin 6) to increase the current comparator threshold, thus

tracking the load current.

When the load is relatively light, the LTC1626 automati-

cally switches to Burst Mode operation. The current loop

is interrupted when the output voltage reaches the desired

regulated value. The hysteretic voltage comparator V trips

the switch and causing the timing capacitor to discharge.

generated. The circuit now enters into sleep mode with the

power MOSFET turned off. In sleep mode, the LTC1626 is

in standby and the load current is supplied by the output

capacitor. All unused circuitry is shut off, reducing quies-

cent current from 1.9mA to 165

µA. When the output

capacitor discharges by the amount of the hysteresis of

the comparator V, the P-channel switch turns on again and

the process repeats itself. During Burst Mode operation,

To avoid the operation of the current loop interfering with

in the gain stage. This prevents the current from increas-

ing until the output voltage has dropped below a minimum

threshold.

In dropout, the P-channel MOSFET is turned on continu-

ously (100% duty cycle) providing low dropout operation

The basic LTC1626 application circuit is shown in Figure

1. External component selection is driven by the load

With the current comparator monitoring the voltage devel-

determines the peak inductor current. Depending upon

the load current condition, the threshold of the compara-

maximum output current of the LTC1626 is:

At a relatively light load, the LTC1626 is in Burst Mode

operation. In this mode, the peak current is set at 25mV/