SC656 Datasheet, PDF (12/17 Pages) Semtech Corporation – Backlight Driver for 7 LEDs with Charge Pump and PWM Control

SC656
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:
• 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 1,
1.5, or 2 times. The charge pump switches at a fixed fre-
quency of 250kHz in 1.5x and 2x modes and is disabled in
1x mode to save power and improve efficiency.
The mode selection circuit automatically selects the 1x,
1.5x, or 2x mode based on circuit conditions such as LED
voltage, input voltage, and load current. The 1x mode is
the most efficient mode, followed by 1.5x 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 (1.5x
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 C1+ and C1- 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
1µF to support the charge pump current requirements.
The device also requires at least 1µF capacitance on the IN
pin and at least 1µF capacitance on the OUT pin to mini-
mize noise and support the output drive requirements of
I_OUT up to 90mA. For output currents higher than 90mA, a
nominal value of 2.2µF is recommended for C_OUT and C_IN.
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 175mA is achieved using 2.2µF
0603 size capacitors for input, output, and bucket
capacitors.
For applications which do not require the full 175mA
output capability of the SC656 , a lower cost and smaller
size capacitor option may be used. The 1µF capacitor in
Table 1 may be used with no loss in accuracy, for up to
90mA of output current.
Table 1 — Capacitor Recommendations
Capacitance
Value of
C_IN = C_OUT
= C₁ = C₂
Size Code
EIA (JIS)
Application I_OUT Limit
2.2µF
0603 (1608)
up to I_OUT = 175mA⁽¹⁾
1.0µF
0402 (1005)
up to I_OUT = 90mA⁽²⁾⁽³⁾
Notes:
(1) 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 1.0µF, may cause excessive
peak-to-peak output ripple, (>120mV), and some loss of accuracy
in 1.5x 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
BANK1 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 BANK1
are set to 0, the sub bank feature is disabled. In this case,
both ENS and ISETS should be tied to GND.
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