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LT3952

3952fb

For more information www.linear.com/LT3952

Figure 9

IVINCOMP

IVINP IVINN

RIVIN

SENSED

CURRENT

LT3952

3952 F09

OPERATION

Input Current Limit

The LT3952 provides adjustable input current limiting

through the IVINP/IVINN/IVINCOMP amplifier. Connect

the IVINP and IVINN sensing terminals across a resistor

in series with the input. A 20x amplified version of the

IVINP–IVINN differential is generated at IVINCOMP. A

capacitor at IVINCOMP provides filtering and averaging

of the input ripple.

As the average IVINP–IVINN differential reaches 60mV,

the IVINCOMP voltage will reach its 1.2V regulation

threshold and the cycle-by-cycle switch current will be

reduced. In this manner, the input current is regulated

to 60mV differential between IVINP and IVINN.

RSENSE mΩ

()=

60mV

IIN LIM

() A

()

For example, a 2.4A input current limit is programmed

with a 25m sense resistor.

Be aware that input current on start-up and PWM dim-

ming is larger than steady state due to the output current

required to fill the output capacitor. It is recommended to

set the input limit appropriately to prevent interruption of

the regulated output.

Input Current Monitor

An input current monitor is available as the voltage on

IVINCOMP, which reflects a voltage equal to 20× the

IVINP-IVINN differential voltage and has a working range

of 0V to 1.2V as the IVINP-IVINN voltage varies from

0mV to 60mV.

IVINCOMP is a high impedance output and should not be

loaded. Averaging of current sense ripple, and compensa-

tion of the feedback loop is achieved using a 1µF capacitor

from IVINCOMP to GND.

Output Voltage Regulation/Limiting

The LT3952 provides a voltage-feedback error amplifier

through the FB pin to provide output voltage regulation

and limiting for open-LED protection.

Inthecaseofanopen-LEDstrand,thecurrentcommanded

by CTRL is never achieved and the device continues to

drivetheoutputvoltagehigher.Ifleftunchecked,thiscould

result in overvoltage damage to the external components

and the power switch itself, and therefore an output volt-

age limit is required.

By connecting FB to a resistor divider between VOUT and

GND, the maximum output voltage regulates at the 1.2V

threshold of the FB pin. The maximum output voltage is

computed as follows:

VOUT V

()=1.2• 1+

⎛

⎝⎜

⎞

⎠⎟

Figure 10

Figure 11

IVINP IVINN

RIVIN

CFLT

RFLT

LT3952

3952 F10

IVINCOMP

VOUT

LT3952

3952 F11

In some cases, such as deep discontinuous conduction,

a large input ripple current may cause the input current

limit to activate early, cutting into the typical regulation

profile. In this situation, it is possible to filter the ripple

down to a smoother level using an external RC network.

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