WM8311 datasheet(101/302 Pages) WOLFSON

101 / 302 page

Pre-Production

WM8311

PP, May 2012, Rev 3.1

101

16 CURRENT SINKS

16.1 GENERAL DESCRIPTION

The WM8311 provides two Current Sinks, ISINK1 and ISINK2. These are programmable constant-

current sinks designed to drive strings of serially connected LEDs, including white LEDs used in

display backlight applications.

The WM8311 Boost Converter, DC-DC4, is designed as a power source for LED strings. Driving

LEDs in this way is particularly power efficient because no series resistor is required. The Boost

Converter can generate voltages higher than the Battery, Wall or USB supply, producing the

necessary combined forward voltages of long LED strings. See Section 15.15.3 for details of DC-DC4

operation.

16.2 CURRENT SINK CONTROL

The configuration of the Current Sinks is described in the following sections.

16.2.1 ENABLING THE SINK CURRENT

In the ON power state, the Current Sinks ISINK1 and ISINK2 can be enabled in software using the

CS1_ENA and CS2_ENA register fields as defined in Table 48. When the Current Sinks are enabled,

the drive current is controlled by the CS1_DRIVE and CS2_DRIVE bits. Note that the Current Sinks

permit current flow only when the applicable CSn_ENA and CSn_DRIVE bits are both set.

The WM8311 Boost Converter, DC-DC4, is the recommended power source for the Current Sinks.

The recommended switch-on sequence is as follows:



Enable Current Sink and Current Drive (CSn_ENA = 1; CSn_DRIVE = 1)



Enable Boost Converter (DC4_ENA = 1)

The status of the Current Sinks in the SLEEP power state are controlled by CS1_SLPENA and

CS2_SLPENA, as described in Table 48. The Current Sinks may either be disabled in SLEEP or

remain under control of the applicable CSn_ENA register bit.

If a Current Sink is disabled in SLEEP, then the applicable CSn_DRIVE bit is automatically reset to 0

as part of the SLEEP transition sequence. Note that the CSn_DRIVE bit will remain reset at 0

following a WAKE transition; the Current Sink can only be re-enabled by writing to the applicable

CSn_DRIVE register bit.

If both Current Sinks are disabled in SLEEP, then DC4 can also be disabled in SLEEP, by setting

DC4_SLPENA = 0, as described in Section 15.4.2. If DC4 is not disabled, then it is important that

CSn_ENA also remains set in the SLEEP power state.

The recommended switch-off sequence for DC-DC4 and the Current Sinks is as follows:



Disable Current Drive (CSn_DRIVE = 0)



Disable Boost Converter (DC4_ENA = 0)



Disable Current Sink (CSn_ENA = 0)

Note that this switch-off sequence is important in order to avoid forward-biasing on-chip ESD

protection diodes.

WM8311 Datasheet(PDF) 101 Page - Wolfson Microelectronics plc

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