LT3757 datasheet(20/38 Pages) AD

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LTC7840

Rev 0

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OPERATION

The SENSE+ and SENSE– Pins

Since the LTC7840 contains leading edge blanking, an

external RC filter is not required for proper operation.

However, if an external filter is used, the filter components

should be placed close to the SENSE+ and SENSE– pins

on the IC, as shown in Figure 10. The positive and nega-

tive sense node traces should then run parallel to each

other to a Kelvin connection underneath the sense resis-

tor, as shown in Figure 11. Sensing current elsewhere on

the board can add parasitic inductance and capacitance

to the current sense element, degrading the information

at the sense pins and making the programmed current

limit unpredictable. Avoid the temptation to connect the

SENSE– line to the ground plane using a PCB via; this

could result in unpredictable behavior.

The sense resistor should be connected to the source

of the power MOSFET and the ground node using short,

wide PCB traces, as shown in Figure 11. Ideally, the bot-

tom terminal of the sense resistors will be immediately

adjacent to the negative terminal of the output capacitor,

since this path is a part of the high di/dt loop formed by

the switch, boost diode, output capacitor and sense resis-

tor. Placement of the inductors is less critical, since the

current in the inductors is a triangle waveform.

Figure 10.

SENSE–

SENSE+

LTC7840

GATE

VOUT

RSENSE

7840 F10

FILTER COMPONENTS

PLACED NEAR

SENSE PINS

VIN

PGND

DRVCC

VIN

Proper Current Sense Filter Component Placement

Figure 11.

MOSFET SOURCE

TO SENSE

FILTER NEXT

TO CONTROLLER

RSENSE

GND

7840 F11

Connecting the SENSE+ and SENSE– Traces

to the Sense Resistor Using a Kelvin Connection

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>10%) as well as other more serious condi-

tions that may overvoltage the output. In such cases, the

external power MOSFET is turned off until the overvoltage

condition is cleared.

Overcurrent Protection

(ILIM1, ILIM2 Pins and Hiccup Mode)

The LTC7840 has ILIM1 and ILIM2 pins to set the maxi-

mum inductor current limit of each channel respectively,

as described in the ILIM1 and ILIM2 pins function. The

voltage on each ILIM pin is used to generate the internal

ITH clamp voltage. Along with the increase of load cur-

rent, ITH voltage will rise to allow higher inductor current

flowing to the load. If the load current keeps increasing,

the ITH voltage will finally reach the clamp voltage and

the inductor current cannot go up any more. This situa-

tion is thought as an overcurrent event by the controller.

If this event lasts for 16 continuous switching periods,

the controller will enter hiccup mode. The ITH voltage is

pulled down to GND and the external power MOSFET is

turned off. The inductor current will gradually reduce to

zero. At this moment, the soft-start capacitor connected

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