WM8351 datasheet(146/330 Pages) WOLFSON

146 / 330 page

WM8351

Production Data

PD, April 2012, Rev 4.5

146

16 CURRENT SINKS (LED DRIVERS)

16.1 GENERAL DESCRIPTION

The WM8351 includes four pins for driving different types of LEDs.

The ISINKA pin provides a programmable constant-current sink designed to drive a string of serially

connected LEDs, including white LEDs used in display backlights or in camera flash applications.

Using ISINKA in conjunction with DC-DC Converter 2 provides a particularly power-efficient way to

drive such LED strings. The ground connection associated with this Current Sink is the SINKGND pin.

ISINKC, ISINKD and ISINKE are regular open-drain outputs. They are alternate functions of the

GPIO10, GPIO11 and GPIO12 pins respectively. These GPIOs are provided on the LINE power

domain; the associated ground connection is the GND pin.

16.2 CONSTANT-CURRENT SINK

ISINKA is a dedicated LED driver pin equipped with a programmable constant current sink. It is

designed to drive a string of serially connected white LEDs such as those used in display backlights

or photo-flash applications. Powering LEDs in this way is particularly power efficient because no

series resistor is required. DC-DC converter 2, operating as a current-controlled voltage source, is an

ideal power source for LED strings. This converter can generate voltages higher than BATT or LINE,

which can overcome the combined forward voltages of long LED strings (e.g. a string of 7 white LEDs

with a forward voltage of 4V requires at least 28V).

16.2.1

ENABLING THE SINK CURRENT

In Active mode, ISINKA can be enabled in software using the CS1_ENA register field defined in Table

96. If required, the Current Sink function may also be controlled by the Hibernate bit.

Note that these control bits do not exist for ISINKC, ISINKD or ISINKE.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R14 (0Eh)

Power mgmt

(7)

CS1_ENA

Current Sink 1 enable (ISINKA pin)

0 = disabled

1 = enabled

R172 (ACh)

Current Sink

Driver A

CS1_HIB_MO

Current Sink 1 behaviour in Hibernate

mode

0 = disable current sink in Hibernate

1 = leave current sink as in Active

Note:

CS1_ENA can be accessed through R14 or through R172. Reading from or writing to either

Table 96 Enabling ISINKA

When ISINKA is used in conjunction with DC-DC Converter 2, the ISINK should always be switched

on before the DC-DC Converter is switched on. Conversely, the DC-DC Converter should always be

switched off before the ISINK is switched off. If high voltages are used, additional external

components may also be needed to protect the WM8351.

16.2.2

PROGRAMMING THE SINK CURRENT

The sink current for ISINKA can be programmed by writing to the CS1_ISEL register bit. The current

steps are logarithmic to match the logarithmic light sensitivity characteristic of the human eye. The

step size is 1.5dB (i.e. the current doubles every four steps).

WM8351 Datasheet(PDF) 146 Page - Wolfson Microelectronics plc

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