LED7707 datasheet(19/48 Pages) STMICROELECTRONICS

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LED7707

Operation description

19/48

5.1.7

Boost current limit

The design of the external components, especially the inductor and the flywheel diode, must

be optimized in terms of size relying on the programmable peak current limit. The LED7707

improves the reliability of the final application giving the way to limit the maximum current

flowing into the critical components. A simple resistor connected between the BILIM pin and

ground sets the desired value. The voltage at the BILIM pin is internally fixed to 1.23 V and

the current limit is proportional to the current flowing through the setting resistor, according

to the following equation:

Equation 7

where

The maximum allowed current limit is 5 A, resulting in a minimum setting resistor

RBILIM > 240 kΩ. The maximum guaranteed RMS current in the power switch is 2 A.

In a boost converter the RMS current through the internal MOSFET depends on both the

input and output voltages, according to equations 8a (DCM) and 8b (CCM).

The current limitation works by clamping the COMP pin voltage proportionally to RBILIM.

Peak inductor current is limited to the above threshold decreased by the slope

compensation contribution.

Equation 8 a

Equation 8 b

5.1.8

Thermal protection

In order to avoid damage due to high junction temperature, a thermal shutdown protection is

implemented. When the junction temperature rises above 150 °C (typ.), the device turns off

both the control logic and the boost converter and holds the FAULT pin low. The LDO is kept

alive and normal operation is automatically resumed after the junction temperature has

been reduced by 30 °C.

BILIM

PEAK

BOOST

1.2 10

⋅

rms

MOS

⋅

()

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

−

OUT

rms

MOS

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