SSL3250A_5

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 05 — 16 December 2009

10 of 26

NXP Semiconductors

SSL3250A

Photo flash dual LED driver

8.3.5 Timed Flash mode

the flash is used in Timed mode (bit 4, TO_DEF = 1 in Timer control register 01h), the

internal timer will switch off the main LED after the preprogrammed maximum time in the

timer control register has expired.

The timer starts when the Flash mode is activated either by the software strobe

(FLASH_STRB bit in register 02h) or by a LOW to HIGH transition of the hardware strobe

(STRB pin) signal.

In timed mode strobing of the flash is edge sensitive, therefore the flash time is

independent of the level of the software or hardware strobe signal. The flash time is set

according to Equation 5:

(5)

Once the Flash time has expired no interrupt will be generated nor will it be flagged in the

status register. A new flash period can be started immediately after the previous timed

flash period has expired.

8.3.6 Flash mode during RF transmit

Although the driver is not equipped with a separate TXMASK pin, the device can operate

like that to lower the current in the main LED in Flash mode during an RF transmission in

a mobile phone application. An external switch can be connected to the resistor

controlling the nominal current value for the Flash mode. By lowering the current in the

main LED, the inductor current and therefore the current drawn from the battery will be

lowered. Reducing the inductor current has to be fast because the inductor current is

reduced within 50

μs after changing the nominal current level to a lower setting. At the end

of the transmission period, the main LED current can be increased again to the nominal

current level. A soft start circuit will increase the inductor current with a limited slope as

defined by the soft start settings. See Section 8.5.

Fig 8.

Flash mode LED current in Direct enable mode

Flash current using direct control mode

Resistor

Value (k

Ω)

50

125

357

014aaa367

200

600

0

70

100

400

300

500

t

flash

820 ms

Register

54.6 ms

×

–

=