PM6600 datasheet(24/43 Pages) STMICROELECTRONICS

24 / 43 page

Operation description

PM6600

24/43

where the first term represents the highest total forward voltage drop over active ROWs and

the second is the voltage drop across the leading generator (400 mV typ.).

The device continues to monitor the voltage drop across all the rows and automatically

switches to the current generator having the lowest voltage drop.

7.2

Over voltage protection

An adjustable Over-Voltage Protection is available. It can be set feeding the OVSEL pin with

a partition of the output voltage. The voltage of the central tap of the divider is thus

compared to a fixed 1.235 V threshold. When the voltage on the OVSEL pin exceeds the OV

threshold, the FAULT pin is tied low (see Section 9 on page 36) and the device is turned off;

this condition is latched and the PM6600 is restarted by toggling the EN pin or by performing

a Power-On Reset (the POR occurs when the LDO output falls below the lower UVLO

threshold and subsequently crosses the upper UVLO threshold during the rising phase of

the input voltage). Normally, the value of the high-side resistors of the divider is in the order

of 100k

Ω to reduce the output capacitor discharge when the boost converter is off (during

the off phase of the dimming cycle).

The OVSEL divider should be a compensated one, with the capacitors C10 (typically in the

100 pF-330 pF range) that improves noise rejection at the OVSEL pin (see Figure 44) and

C13 (typically 22 pF) that avoids OVP fault detection when a row is open.

The following formula permits to properly select the OVP threshold, according to the VOUT

value and considering the worst case:

Equation 2

where

Equation 3

VOVP is the Over-Voltage Protection threshold

VROWx,FAULT is the Shorted LED threshold

VROW_MAX is the maximum voltage drop across the current generators, measured in the

ROWx pin with the leds' series with minimum VF_WLED: Forward Voltage of the single LED.

)

(

MAX

ROW

FAULT

ROWx

OUT

OVP

OUT

−

WLED

series

WLED

OUT

⋅

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