LTC3124 datasheet(16/36 Pages) AD

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LT3120

Rev. 0

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CURRENT LIMIT AND ZERO CURRENT DETECTION

The LT3120 incorporates inductor current limiting. The

average current mode control loop clamps the average

current in the inductor to 9.5A (typical). This ensures a

smooth transition into current limit and protects against

current levels beyond the capability of the IC.

An additional maximum peak current limit of 12.6A (typi-

cal) limits the instantaneous current in the inductor and

may be triggered briefly on hard output shorts prior to the

average current loop responding.

The LT3120 incorporates dedicated zero current detection

comparators to minimize reverse current during switch-

ing and provide discontinuous mode operation. The zero

currentdetectionthresholdsareadjustedbasedonoperat-

ing conditions to avoid subharmonic switching and may

result in small amounts of negative current under some

conditions. When the part is being controlled by the MPPC

control loop, the zero cross comparator threshold is set to

approximately 150mA to help prevent any reverse current

from discharging the output storage element.

SOFT-START

TheLT3120soft-startcircuitminimizesinputcurrenttran-

sients and output voltage overshoot on initial power-up.

The required timing components for soft-start are internal

to the LT3120 and produce a typical soft-start duration

of 6ms. The internal soft-start circuit slowly ramps the

command signal to the current mode control loop (VC pin

voltage). In doing so, the inductor current is also slowly

increased, starting from zero. Soft-start is reset by the VCC

UVLO, the EN/UVLO pin accurate enable comparator, and

thermal shutdown.

MAXIMUM POWER POINT CONTROL (MPPC)

The MPPC input of the LT3120 can be used with an

optional external voltage divider to dynamically adjust

the commanded inductor current in order to maintain

a minimum input voltage. This is primarily useful when

using resistive sources, such as photovoltaic panels, to

maximize input power transfer and prevent VIN from drop-

ping too low under load. Referring to Figure 5, the MPPC

pin is internally connected to the noninverting input of a

transconductance amplifier. If the MPPC pin voltage falls

below the reference voltage, the output of the amplifier

reduces the commanded average inductor current (VC

pin voltage) to reduce the input current and regulate VIN

to the programmed minimum voltage.

OPERATION

CC2

RC2

3120 F05

PVIN 795mV

MPPC

CIN

VIN

LT3120

–

Figure 5. MPPC Pin Configuration

External compensation may also be required and is gener-

ally provided by a series resistor and capacitor connected

between the MPPC pin and GND. This compensation

network is in parallel with the lower resistor of the VIN

voltage divider network.

PROGRAMMABLE OUTPUT CURRENT

The LT3120 includes an optional programmable current

limit that can be used to control current to the output load.

The current limit circuit requires an external sense resistor

RSEN connected between ISP and ISN and a programming

resistor from PROG to ground.

The current sense amplifier outputs a current from the

PROG pin that is proportional to the voltage between pins

ISP and ISN. Output current limiting will begin when the

voltage on the PROG pin reaches 795mV.

The current output from the PROG pin is equal to Equation 2.

IPROG = 2µA/mV • RSEN • IOUT

(2)

Independent value selection for RSEN and RPROG allow

designs to trade-off current sense accuracy and power

loss in the sense element. The output current is pro-

grammed using Equation 3.

ILIMIT = 795V/(2 • RSEN • RPROG )

(3)

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