LED7708 datasheet(27/53 Pages) STMICROELECTRONICS

27 / 53 page

LED7708

Recommended operating conditions

Doc ID 022698 Rev 1

27/53

Detailed information about the MOSFET gate driver can be found in Section 5: Electrical

characteristics.

The current flowing through the power MOSFET is sensed during the on-phase by

measuring the voltage drop across an external sensing resistor (connected between the

source of the MOSFET and PGND). The overcurrent condition is detected by comparing the

sensed voltage with an internal threshold. This threshold can be set in the 100 mV - 300 mV

range according to the resistor connected between the CLIM pin and ground. The sensing

resistors are often available in few values: once a suitable one has been selected, the

resistor at the CLIM pin is calculated according to the following equation:

Equation 9

where KC1 = 1.63 x 10

-5 V and K

C2 = 3.1 x 10

-2 A.

The maximum allowed peak current depends on the external MOSFET capability.

The actual maximum peak current is given by the above equations as long as duty cycle is

below 35% and decreases as duty cycle increases beyond 35%, depending on the slope

compensation setting. If an overcurrent (OC) condition occurs, the boost controller turns-off

the external MOSFET and skips subsequent switching cycles until the current is above the

programmed level.

In the case of a light load condition, the COMP pin voltage may fall below 0.8 V (typ.). In this

case the boost controller stops switching until the error amplifier forces the COMP voltage to

rise. The above voltage at the COMP pin corresponds to about 25% of full current limit.

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

input and output voltages, according to Equation 10 (DCM) and 11 (CCM).

Equation 10

Equation 11

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

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

compensation contribution.

8.4

Current generators section

The LED-driving section consists of 16 current generators connected to the internal control

logic. Each channel can sustain up to 40 V and sink up to 85 mA. The brightness of the

LEDs connected to the LED7708 is controlled by switching the current between a maximum

value (on-phase) and a minimum one (off-phase) at a given dimming frequency (PWM

control).

CLIM

SNS

MOS OCP

⋅

()

---------------------------------------------------------------------

MOS rms

⋅

---------------------- D

----

()

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

−

⎟⎟⎠

⎞

⎜⎜⎝

⎛

⋅

−

OUT

rms

MOS

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