WM8351 datasheet(324/330 Pages) WOLFSON

324 / 330 page

WM8351

Production Data

PD, April 2012, Rev 4.5

324

29.4.2

DC-DC (STEP-UP) CONVERTER - CONSTANT CURRENT MODE

Constant current mode is selected by setting DC2_FBSRC[1:0] = 01, as described in Section 14.6.4.

This results in the DC Converter controlling the current at ISINKA. The recommended connections to

the DC-DC (Step-Up) Converter in this mode are illustrated in Figure 90.

Figure 90 DC-DC (Step-Up) Converter External Components - Constant Current Mode

In the constant current mode, the DC-DC Converter output voltage is controlled by the WM8351 in

order to achieve the required current in ISINKA. The required current is set by the CS1_ISEL register

field, as described in Section 16.2.2. A typical application for this mode would be a white LED driver,

where several LEDs are connected in series to achieve uniform brightness.

The DC-DC (Step-Up) Converters are capable of generating output voltages of up to 30V. The

maximum output voltage is determined by the two external resistors R1 and R2, which form a

resistive divider between load connection and the voltage feedback pin FB2 or FB5.

The choice of resistors R1 and R2 follows the same equations as for the constant voltage mode (see

Section 29.4.1). Note that, in constant current mode, the resistors determine the maximum output

voltage. The actual voltage will be determined by the selected ISINK current, subject to the device

limits.

The choice of Capacitors, Inductor and FET in constant current mode is the same as for the constant

voltage mode; see Section 29.4.4 for specific recommended component details.

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.

WM8351 Datasheet(PDF) 324 Page - Wolfson Microelectronics plc