LT1933 datasheet(6/16 Pages) LINER

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LT1933

1933f

APPLICATIO S I FOR ATIO

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1%

resistors according to:

R1 = R2(VOUT/1.245 – 1)

R2 should be 20k or less to avoid bias current errors.

Reference designators refer to the Block Diagram.

Input Voltage Range

The input voltage range for LT1933 applications depends

on the output voltage and on the absolute maximum

ratings of the VIN and BOOST pins.

The minimum input voltage is determined by either the

LT1933’s minimum operating voltage of ~3.35V, or by its

maximum duty cycle. The duty cycle is the fraction of time

that the internal switch is on and is determined by the input

and output voltages:

DC = (VOUT + VD)/(VIN – VSW + VD)

where VD is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

(~0.4V at maximum load). This leads to a minimum input

voltage of:

VIN(MIN) = (VOUT + VD)/DCMAX – VD + VSW

with DCMAX = 0.88

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BOOST pins and by the

minimum duty cycle DCMIN = 0.08 (corresponding to a

minimum on time of 130ns):

VIN(MAX) = (VOUT + VD)/DCMIN – VD + VSW

Note that this is a restriction on the operating input

voltage; the circuit will tolerate transient inputs up to the

absolute maximum ratings of the VIN and BOOST pins.

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is:

L = 5 (VOUT + VD)

where VD is the voltage drop of the catch diode (~0.4V) and

L is in µH. With this value the maximum load current will

be above 500mA. The inductor’s RMS current rating must

be greater than your maximum load current and its satu-

ration current should be about 30% higher. For robust

operation in fault conditions the saturation current should

OPERATIO

The LT1933 is a constant frequency, current mode step

down regulator. A 500kHz oscillator enables an RS flip-

flop, turning on the internal 750mA power switch Q1. An

amplifier and comparator monitor the current flowing

between the VIN and SW pins, turning the switch off when

this current reaches a level determined by the voltage at

VC. An error amplifier measures the output voltage through

an external resistor divider tied to the FB pin and servos the

VC node. If the error amplifier’s output increases, more

current is delivered to the output; if it decreases, less

current is delivered. An active clamp (not shown) on the VC

node provides current limit. The VC node is also clamped

to the voltage on the SHDN pin; soft-start is implemented

by generating a voltage ramp at the SHDN pin using an

external resistor and capacitor.

An internal regulator provides power to the control cir-

cuitry. This regulator includes an undervoltage lockout to

prevent switching when VIN is less than ~3.35V. The SHDN

pin is used to place the LT1933 in shutdown, disconnect-

ing the output and reducing the input current to less than

2µA.

The switch driver operates from either the input or from

the BOOST pin. An external capacitor and diode are used

to generate a voltage at the BOOST pin that is higher than

the input supply. This allows the driver to fully saturate the

internal bipolar NPN power switch for efficient operation.

The oscillator reduces the LT1933’s operating frequency

when the voltage at the FB pin is low. This frequency

foldback helps to control the output current during startup

and overload.

(Refer to Block Diagram)

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