LT3494EDDB データシートの表示（PDF） - Linear Technology

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LT3494EDDB

Micropower Low Noise Boost Converters with Output Disconnect

Linear Technology 

LT3494/LT3494A

APPLICATIONS INFORMATION

Connecting the Load to the CAP Node

The efficiency of the converter can be improved by con-

necting the load to the CAP pin instead of the VOUT pin.

The power loss in the PMOS disconnect circuit is then

made negligible. By connecting the feedback resistor to

the VOUT pin, no quiescent current will be consumed in the

feedback resistor string during shutdown since the PMOS

transistor will be open (see Figure 3). The disadvantage

of this method is that the CAP node cannot go to ground

during shutdown, but will be limited to around a diode

drop below VCC. Loads connected to the part should only

sink current. Never force external power supplies onto

the CAP or VOUT pins. The larger value output capacitor

(2.2μF to 10μF) should be placed on the node to which

the load is connected.

1

8

SW

3

VCC

CAP

7

VOUT

LT3494

5

SHDN

6

FB

4

2

CTRL GND

C1

3494 F03

ILOAD

Figure 3. Improved Efficiency

Maximum Output Load Current

The maximum output current of a particular LT3494/

LT3494A circuit is a function of several circuit variables.

The following method can be helpful in predicting the

maximum load current for a given circuit:

Step 1: Calculate the peak inductor current:

IPK

=

ILIMIT

+

VIN

•

400

L

•

10–9

amps

where ILIMIT is 0.180A and 0.350A for the LT3494 and

LT3494A 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

• 150 • 10–9

amps

where VOUT is the desired output voltage.

If the inductor ripple current is greater than the peak cur-

rent, 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 • 0.75

VOUT

amps

Step 5: Derate output current:

IOUT = IOUT(NOM) • 0.7 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 volt-

age while the output capacitor is discharged, a higher

level of inrush current may flow through the inductor

and integrated 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 internal diode is designed to handle

such events, the inrush current should not be allowed to

exceed 1A. 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. The LT3494A circuits experience

higher levels of current during start-up and steady-state

operation. An external diode placed from the SW pin to

3494fb

9

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