sc656 Semtech Corporation, sc656 Datasheet - Page 12

• Manufacturer Part Number: sc656
• Description: Backlight Driver For 7 Leds With Charge Pump And Pwm Control
• Manufacturer: Semtech Corporation

Applications Information
General Description
This design is optimized for handheld applications sup-
plied from a single Li-Ion cell and includes the following
key features:
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
•
•
•
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.
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.
Notes:
() Note that 2.2µF in the 0402 size is not equivalent to 2.2µF in the
(2) Larger size capacitors may be substituted.
(3) Exceeding 90mA, or using less than .0µF, may cause excessive
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.
OUT
Capacitance
up to 90mA. For output currents higher than 90mA, a
0603 size, so 0402 may not be substituted for this application.
peak-to-peak output ripple, (>20mV), and some loss of accuracy
in .5x mode.
C
Value of
= C
IN
2.2µF
.0µF
= C
Table 1 — Capacitor Recommendations
1
= C
OUT
2
0603 (608)
0402 (005)
Size Code
EIA (JIS)
Application I
up to I
up to I
OUT
OUT
= 90mA
= 75mA
OUT
OUT
and C
Limit
(2)(3)
()
IN
2
.