SC656

2

General Description

This design is optimized for handheld applications sup-

plied from a single Li-Ion cell and includes the following

key features:

A high efficiency fractional charge pump that

supplies power to all LEDs

Seven matched current sinks that control LED

backlighting current, providing 500µA to 25mA

per LED

Two LED bank options with independent current

settings and enable pins with PWM control of

LED brightness.

High Current Fractional Charge Pump

The backlight outputs are supported by a high efficiency,

high current fractional charge pump output at the OUT

pin. The charge pump multiplies the input voltage by ,

.5, or 2 times. The charge pump switches at a fixed fre-

quency of 250kHz in .5x and 2x modes and is disabled in

x mode to save power and improve efficiency.

The mode selection circuit automatically selects the x,

.5x, or 2x mode based on circuit conditions such as LED

voltage, input voltage, and load current. The x mode is

the most efficient mode, followed by .5x and 2x modes.

Circuit conditions such as low input voltage, high output

current, or high LED voltage place a higher demand on

the charge pump output. A higher numerical mode (.5x

or 2x) may be needed momentarily to maintain regulation

at the OUT pin during intervals of high demand. The

charge pump responds to momentary high demands,

setting the charge pump to the optimum mode to deliver

the output voltage and load current while optimizing effi-

ciency. Hysteresis is provided to prevent mode toggling.

The charge pump requires two bucket capacitors for

proper operation. One capacitor must be connected

between the C+ and C- pins and the other must be con-

nected between the C2+ and C2- pins as shown in the

Typical Application Circuit diagram. These capacitors

should be equal in value, with a minimum capacitance of

µF to support the charge pump current requirements.

The device also requires at least µF capacitance on the IN

pin and at least µF capacitance on the OUT pin to mini-

mize noise and support the output drive requirements of

•

•

•

I

nominal value of 2.2µF is recommended for C

Capacitors with X7R or X5R ceramic dielectric are strongly

recommended for their low ESR and superior tempera-

ture and voltage characteristics. Y5V capacitors should

not be used as their temperature coefficients make them

unsuitable for this application.

Capacitor Recommendations

The full rated output of 75mA is achieved using 2.2µF

0603 size capacitors for input, output, and bucket

capacitors.

For applications which do not require the full 75mA

output capability of the SC656 , a lower cost and smaller

size capacitor option may be used. The µF capacitor in

Table  may be used with no loss in accuracy, for up to

90mA of output current.

Table 1 — Capacitor Recommendations

Capacitance

Value of

C

= C

Size Code

EIA (JIS)

Application I

2.2µF

0603 (608)

up to I

()

.0µF

0402 (005)

up to I

(2)(3)

Notes:

() Note that 2.2µF in the 0402 size is not equivalent to 2.2µF in the

0603 size, so 0402 may not be substituted for this application.

(2) Larger size capacitors may be substituted.

(3) Exceeding 90mA, or using less than .0µF, may cause excessive

peak-to-peak output ripple, (>20mV), and some loss of accuracy

in .5x mode.

Bank Control Options

The backlight drivers can be configured as a single bank

or as two independently controlled banks. The configu-

ration of the banks is determined by the BANK2 and

BANK pins as described in Table 2. The ENM and ISETM

pins control the brightness of LEDs assigned to the main

bank, and the ENS and ISETS pins allow the sub bank

current to be set independently as described in the fol-

lowing section. Note that when both BANK2 and BANK

are set to 0, the sub bank feature is disabled. In this case,

both ENS and ISETS should be tied to GND.

Applications Information