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LT3957

3957f

APPLICATIONS INFORMATION

The Internal Power Switch Current

For control and protection, the LT3957 measures the

internal power MOSFET current by using a sense resistor

(RSENSE) between GND and the MOSFET source. Figure 3

shows a typical waveform of the internal switch current

(ISW).

Due to the current limit (minimum 5A) of the internal power

switch, the LT3957 should be used in the applications

that the switch peak current ISW(PEAK) during steady state

normal operation is lower than 5A by a sufﬁcient margin

(10% or higher is recommended).

The LT3957 switching controller incorporates 100ns

timing interval to blank the ringing on the current sense

signal across RSENSE immediately after M1 is turned on.

This ringing is caused by the parasitic inductance and

capacitance of the PCB trace, the sense resistor, the diode,

and the MOSFET. The 100ns timing interval is adequate

for most of the LT3957 applications. In the applications

that have very large and long ringing on the current sense

signal, a small RC ﬁlter can be added to ﬁlter out the excess

ringing. Figure 4 shows the RC ﬁlter on the SENSE1 and

SENSE2 pins. It is usually sufﬁcient to choose 22Ω for

RFLT and 2.2nF to 10nF for CFLT. Keep RFLT’s resistance

low. Remember that there is 65μA (typical) ﬂowing out of

the SENSE2 pin. Adding RFLT will affect the internal power

switch current limit threshold:

ISW _ILIM = 1−

65µA •RFLT

48mV

⎛

⎝⎜

⎞

⎠⎟ •5A

On-Chip Power Dissipation and Thermal Lockout (TLO)

The on-chip power dissipation of LT3957 can be estimated

using the following equation:

PIC ≈ I2SW • D • RDS(ON) + V2PEAK • ISW • ƒ • 200pF/A +

VIN • (1.6mA + ƒ • 10nC)

where RDS(ON) is the internal switch on-resistance which

can be obtained from the Typical Performance Characteris-

tics section. VSW(PEAK) is the peak switch off-state voltage.

The maximum power dissipation PIC(MAX) can be obtained

by comparing PIC across all the VIN range at the maximum

output current . The highest junction temperature can be

estimated using the following equation:

TJ(MAX) ≈ TA + PIC(MAX) • 42°C/W

It is recommended to measure the IC temperature in steady

state to verify that the junction temperature limit is not

exceeded. A low switching frequency may be required to

ensure TJ(MAX) does not exceed 125°C.

If LT3957 die temperature reaches thermal lockout

threshold at 165°C (typical), the IC will initiate several

protective actions. The power switch will be turned off.

A soft-start operation will be triggered. The IC will be en-

abled again when the junction temperature has dropped

by 5°C (nominal).

Figure 3. The Switch Current During a Switching Cycle

3957 F03

ISW(PEAK)

ISW

DTS

Figure 4. The RC Filter on SENSE1 Pin and SENSE2 Pin

3957 F04

LT3957

RFLT

CFLT

SENSE2

SGND

SENSE1

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