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LTC1574_1

High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode

Linear Technology 

LTC1574

LTC1574-3.3/LTC1574-5

PI FU CTIO S

NC (Pins 1, 8, 9, 16): No Connection.

GND (Pins 2, 4, 13, 15): Ground.

SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and

Cathode of Schottky Diode.

VIN (Pin 5): Input Supply Voltage. It must be decoupled

close to ground (Pin 4).

IPGM (Pin 6): This pin selects the current limit of the

P-channel switch. With IPGM = VIN, the current trip point is

600mA and with IPGM = 0V, the current trip point is

reduced to 340mA.

SHDN (Pin 7): Pulling this pin to ground keeps the internal

switch off and puts the LTC1574 in micropower shutdown.

VOUT or VFB (Pin 10): For the LTC1574, this pin connects

to the main voltage comparator input. On the LTC1574-5

and LTC1574-3.3, this pin goes to an internal resistive

divider which sets the output voltage.

LBOUT (Pin 11): Open drain of an N-Channel Pull-Down.

This pin will sink current when (Pin 12) LBIN goes below

1.25V.

LBIN (Pin 12): The (–) Input of the Low-Battery Voltage

Comparator. The (+) input is connected to a reference

voltage of 1.25V.

APPLICATIO S I FOR ATIO

Operating Frequency and Inductor

Since the LTC1574 utilizes a constant off-time architecture,

its operating frequency is dependent on the value of VIN. The

frequency of operation can be expressed as:

( ) f

=

1

tOFF





VIN − VOUT 

VIN + VD 

Hz

where tOFF = 4µs and VD is the voltage drop across the

internal Schottky diode. Note that the operating frequency

is a function of the input and output voltage.

Although the size of the inductor does not affect the fre-

quency or inductor peak current, it does affect the ripple

current. The peak-to-peak ripple current is given by:

( ) IRIPPLE

=

4

•

10−6





VOUT +

L

VD





AP-P

When choosing a small inductor, core loss will increase due

to higher ripple current. Therefore, a low ESR output

capacitor has to be used.

Short-Circuit Protection

The LTC1574 is protected from output short circuits by its

internal current limit. Depending on the condition of the

IPGM pin, the limit is either set to 340mA or 600mA. In

addition, the off-time of the switch is increased to allow the

inductor current to decay far enough to prevent any current

build-up (see Figure 1).

IPGM = VIN

IPGM = 0

GND

L = 100µH

VIN = 13.5V

20µs/DIV

1574 • F01

Figure 1. Inductor Current with Output Shorted

Low-Battery Detector

The low-battery indicator senses the input voltage through

an external resistive divider. This divided voltage connects

to the “–” input of a voltage comparator (Pin 12) which is

compared with a 1.25V reference voltage. With the current

4

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