LT3503 Просмотр технического описания (PDF) - Linear Technology

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LT3503

1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3503

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 20.0k 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 LT3503 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

LT3503’s minimum operating voltage of 3.6V, 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 = 0.81 (0.76 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 = 0.29. 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

8

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 2.2MHz.

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).

Minimum On Time

The part will still regulate the output at input voltages that

exceed VIN(PS) (up to 20V), but the output voltage ripple

increases. Figure 1 illustrates switching waveforms in

continuous mode for a 0.78V output application near

VIN(PS) = 6V.

As the input voltage is increased, the part is required to

switch for shorter periods of time. Delays associated with

turning off the power switch dictate the minimum on time

of the part. The minimum on time for the LT3503 is

~130ns. Figure 2 illustrates the switching waveforms

when the input voltage is increased to VIN = 14V.

VSW

10V/DIV

IL

1A/DIV

VOUT

20mV/DIV

COUT = 47µF

VOUT = 0.78V

VIN = 7V

ILOAD = 1.1A

L = 1.1µH

1µs/DIV

Figure 1

3503 F01

VSW

10V/DIV

IL

1A/DIV

VOUT

20mV/DIV

COUT = 47µF

VOUT = 0.78V

VIN = 14V

ILOAD = 1.1A

L = 1.1µH

1µs/DIV

Figure 2

3503 F02

3503f

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