LT3493E-3 查看數據表（PDF） - Linear Technology

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LT3493E-3

1.2A, 750kHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3493-3

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

0.78V

–

1⎞⎠⎟

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

Reference designators refer to the Block Diagram.

An optional phase lead capacitor of 22pF between VOUT

and FB reduces light-load output ripple.

Input Voltage Range

The input voltage range for LT3493-3 applications de-

pends 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

LT3493-3’s minimum operating voltage of 6.8V, 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 = 0.91 (0.88 over temperature).

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BOOST pins. For con-

stant-frequency operation, the maximum input voltage is

determined by the minimum duty cycle, DCMIN = 0.10. If

the duty cycle requirement is less than DCMIN, the part will

enter pulse-skipping mode. The onset of pulse-skipping

occurs at:

VIN(PS)

=

VOUT + VD

DCMIN

–

VD

+

VSW

In pulse-skipping mode, the part skips pulses to control

the inductor current and regulate the output voltage,

possibly producing a spectrum of frequencies below

750kHz.

Note that this is a restriction on the operating input voltage

to remain in constant-frequency operation; the circuit will

tolerate transient inputs up to the absolute maximum

ratings of the VIN and BOOST pins when the output is in

regulation. The input voltage should be limited to VIN(PS)

during overload conditions (short-circuit or start-up).

VSW

20V/DIV

IL

0.5A/DIV

VOUT

200mV/DIV

AC COUPLED

COUT = 10µF

VOUT = 3V

VIN = 30V

ILOAD = 0.75A

L = 10µH

2µs/DIV

Figure 1

3493-3 F01

3493-3f

8

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