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LTC1701

OPERATIO

The LTC1701 uses a contant off-time, current mode

architecture. The operating frequency is then determined

by the off-time and the difference between VIN and VOUT.

To optimize efficiency, the LTC1701 automatically switches

between continuous and Burst Mode operation.

The output voltage is set by an external divider returned to

the VFB pin. An error amplfier compares the divided output

voltage with a reference voltage of 1.25V and adjusts the

peak inductor current accordingly.

Main Control Loop

During normal operation, the internal PMOS switch is

turned on when the VFB voltage is below the reference

voltage. The current into the inductor and the load in-

creases until the current limit is reached. The switch turns

off and energy stored in the inductor flows through the

external Schottky diode into the load. After the constant

off-time interval, the switch turns on and the cycle repeats.

The peak inductor current is controlled by the voltage on

the ITH/RUN pin, which is the output of the error

amplifier.This amplifier compares the VFB pin to the 1.25V

reference. When the load current increases, the FB voltage

decreases slightly below the reference. This decrease

causes the error amplifier to increase the ITH/RUN voltage

until the average inductor current matches the new load

current.

The main control loop is shut down by pulling the ITH/RUN

pin to ground. When the pin is released an external resistor

is used to charge the compensation capacitor. When the

voltage at the ITH/RUN pin reaches 0.8V, the main control

loop is enabled and the error amplifier drives the ITH/RUN

pin. Soft-start can be implemented by ramping the voltage

on the ITH/RUN pin (see Applications Information sec-

tion).

Low Current Operation

When the load is relatively light, the LTC1701 automati-

cally switches to Burst Mode operation in which the

internal PMOS switch operates intermittently based on

load demand. The main control loop is interrupted when

the output voltage reaches the desired regulated value.

The hysteretic voltage comparator trips when ITH/RUN is

below 1.5V, shutting off the switch and reducing the

power consumed. The output capacitor and the inductor

supply the power to the load until the output voltage drops

slightly and the ITH/RUN pin exceeds 1.5V, turning on the

switch and the main control loop which starts another

cycle.

Dropout Operation

In dropout, the internal PMOS switch is turned on continu-

ously (100% duty cycle) providing low dropout operation

with VOUT at VIN. Since the LTC1701 does not incorporate

an under voltage lockout, care should be taken to shut

down the LTC1701 for VIN < 2.5V.

APPLICATIO S I FOR ATIO

The basic LTC1701 application circuit is shown in

Figure 1. External component selection is driven by the

load requirement and begins with the selection of L1. Once

L1 is chosen, the Schottky diode D1 can be selected

followed by CIN and COUT.

L Selection and Operating Frequency

The operating frequency is fixed by VIN, VOUT and the

constant off-time of about 500ns. The complete expres-

sion for operating frequency is given by:

fO =

−

















OUT

OFF

Although the inductor does not influence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current

∆IL decreases with

higher inductance and increases with higher VIN or VOUT:

∆=

−

















OUT

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