LT3495(RevA) Просмотр технического описания (PDF) - Linear Technology

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LT3495
(Rev.:RevA)

650mA/350mA Micropower Low Noise Boost Converter with Output Disconnect

Linear Technology 

LT3495/LT3495B/

LT3495-1/LT3495B-1

APPLICATIONS INFORMATION

Maximum Output Load Current

The maximum output current of a particular LT3495 series

circuit is a function of several circuit variables. The fol-

lowing method can be helpful in predicting the maximum

load current for a given circuit:

Step 1: Calculate the peak inductor current:

IPK

= ILIMIT

+

VIN

• 200

L

• 10 –9

amps

where ILIMIT is 0.65A and 0.35A for LT3495/LT3495B and

LT3495-1/LT3495B-1 respectively. L is the inductance

value in Henrys and VIN is the input voltage to the boost

circuit.

Step 2: Calculate the inductor ripple current:

( ) IRIPPLE =

VOUT + 1– VIN

L

• 200 •10–9

amps

where VOUT is the desired output voltage. If the inductor

ripple current is greater than the peak current, then the

circuit will only operate in discontinuous conduction mode.

The inductor value should be increased so that IRIPPLE < IPK.

An application circuit can be designed to operate only in

discontinuous mode, but the output current capability

will be reduced.

Step 3: Calculate the average input current:

IIN( AVG)

=

IPK

–

IRIPPLE

2

amps

Step 4: Calculate the nominal output current:

IOUT(NOM)

=

IIN(AVG) • VIN

VOUT

•

0.8

amps

Step 5: Derate output current:

IOUT = IOUT(NOM) • 0.8 amps

For low output voltages the output current capability will

be increased. When using output disconnect (load cur-

rent taken from VOUT), these higher currents will cause

the drop in the PMOS switch to be higher resulting in

reduced output current capability than those predicted

by the preceding equations.

Inrush Current

When VCC is stepped from ground to the operating voltage

while the output capacitor is discharged, a higher level of

inrush current may flow through the inductor and Schottky

diode into the output capacitor. Conditions that increase

inrush current include a larger more abrupt voltage step

at VIN, a larger output capacitor tied to the CAP pin and an

inductor with a low saturation current. While the chip is

designed to handle such events, the inrush current should

not be allowed to exceed 1.5A. For circuits that use output

capacitor values within the recommended range and have

input voltages of less than 5V, inrush current remains low,

posing no hazard to the device. In cases where there are

large steps at VCC (more than 5V) and/or a large capacitor

is used at the CAP pin, inrush current should be measured

to ensure safe operation.

Soft-Start

By connecting the SHDN and CTRL pins as shown in

Figure 4, using an RC filter at the CTRL pin to limit the

start-up current, the LT3495 is able to achieve soft-start.

The small bias current of the CTRL pin allows using a

small capacitor for a large RC time constant. The soft-

start waveform is shown in Figure 5. The soft-start time

CHIP ENABLE

RCTRL

CCTRL

SW CAP

VCC VOUT

LT3495

SHDN FB

CTRL GND

3495 F04

Figure 4. Soft-Start Circuitry

SHDN VOLTAGE

5V/DIV

INDUCTOR CURRENT

500mA/DIV

CTRL VOLTAGE

2V/DIV

VOUT VOLTAGE

5V/DIV

VCC = 3.6V

VOUT = 16V

500µs/DIV

3495 F05

Figure 5. Soft-Start Waveform

3495b1b1fa

11

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