5

LTC1701/LTC1701B

OPERATIO

The LTC1701 uses a contant off-time, current mode archi-

tecture. The operating frequency is then determined by the

The output voltage is set by an external divider returned to

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

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

reference. When the load current increases, the FB voltage

decreases slightly below the reference. This decrease

until the average inductor current matches the new load

current.

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

is used to charge the compensation capacitor. When the

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

tion).

Low Current Operation

To optimize efficiency when the load is relatively light, the

LTC1701 automatically switches to Burst Mode operation

in which the internal PMOS switch operates intermittently

based on load demand. The main control loop is inter-

rupted when the output voltage reaches the desired regu-

lated value. The hysteretic voltage comparator trips when

ing the power consumed. The output capacitor and the

inductor supply the power to the load until the output

turning on the switch and the main control loop which

starts another cycle.

For reduced output ripple, the LTC1701B doesn't use

Burst Mode operation and operates continuously down to

very low currents where the part starts skipping cycles.

Dropout Operation

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

ously (100% duty cycle) providing low dropout operation

an under voltage lockout, care should be taken to shut

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

L Selection and Operating Frequency

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

sion for operating frequency is given by:

−

+

















VV

VV

T

IN

OUT

IN

D

OFF

1

Although the inductor does not influence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current

∆=

−









+

+









I

VV

fL

VV

VV

L

IN

OUT

OUT

D

IN

D