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LT3952

3952fb

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OPERATION

The second auxiliary loop is the input current limit us-

ing the IVINN, IVINP, and IVINCOMP pins. Similar to the

output (LED) current loop, the input current is sensed

as the differential voltage across a sense resistor. As the

average IVINP/IVINN differential voltage exceeds 60mV,

IVINCOMP reaches its 1.2V threshold, and VC is reduced

in order to limit the input current to the 60mV threshold.

INTVCC Bypass Capacitor

INTVCC is the internal power supply for the IC, and sup-

plies the gate drive to the internal power switch. A bypass

capacitor is required from INTVCC to GND for stability

and filtering of the switching noise. For best results, use

a ceramic capacitor of 2.2µF or greater, and place the

capacitor as close to the IC as possible.

VREF Bypass Capacitor

VREF is the 2V reference output and can be bypassed with

100nFtoGND.Forbestresults,placethebypasscapacitor

close to the IC and away from the noisy switching nodes.

Programming the Turn-On/Turn-Off Thresholds

with the EN/UVLO Pin

The LT3952 provides an adjustable VINundervoltagelock-

out(UVLO)functionusingtheEN/UVLOpin.TheEN/UVLO

function provides a precision 1.23V falling threshold with

75mVinternalhysteresisaswellasa2Apull-downcurrent

in the off state to provide additional, user-programmable

hysteresis. This pin can also be driven with a logic level

greater than 1.5V or tied to VIN for always-on operation.

In order to program an external UVLO, tie the EN/UVLO

pin to a resistor divider between VIN and GND. The divider

ratio determines the baseline turn-off/turn-on thresholds

and the value of the upper resistor determines the ad-

ditional hysteresis.

VIN

EN/UVLO

LT3952

3952 F01

Figure 1

The most important (and precise) threshold is the turn-

off threshold from VIN falling. The following equations

determine the resistor values:

VUV(FALLING) =1.23• 1+

⎛

⎝⎜

⎞

⎠⎟

VUV(RISING) =1.305• 1+

⎛

⎝⎜

⎞

⎠⎟

+ 2µA •R1

For example, a 12V falling UVLO with roughly 10% total

hysteresis requires 185k for R1. The closest standard

value is 187k, therefore:

R1 = 187k

R2 = 21.5k

VUV(FALLING) = 11.93V

VUV(RISING) = 13.03V

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