SM8131B Datasheet, PDF (14/21 Pages) Nippon Precision Circuits Inc – The SM8131B is a charge pump DC/DC converter that switches between 1 and 1.5 automatically.

SM8131B
VIN Input Voltage Range
The V_IN minimum input voltage value to ensure rated drive current flows in each LED is determined by the
I_LED setting (largest setting if divided into 2 groups), the total load current I_OUT on pin VOUT, and the LED
forward-direction voltage drop “VF”, as given in the following equations:
(1) VF ≤ 1.5 × VIN − 10 × I_OUT − 8 × I_LED
(2) VIN ≥ (VF + 10 × I_OUT + 8 × I_LED)/1.5
where 1.5 is the charge pump boost factor, 10 is the VOUT output impedance, and 8 is the LED drive circuit
internal resistance.
For example, if V_IN = 3.5V, I_LED = 20mA, and I_OUT = 120mA, then VF is given by equation as follows:
1.5 × 3.5 − 10 × 0.12 − 8 × 0.02 = 3.89 [V]
Thus if LEDs with VF of 3.89V or lower rating at I_LED = 20mA are used, the device will provide sufficient
drive current. If V_IN falls and the drive LED VF exceeds the calculated value, V_IN will take I_LED down with it
below the required setting.
The relationship in equation is shown graphically for values of I_OUT = 30/60/90/120mA.
5.5
IOUT = 30mA
IOUT = 60mA
5.0
IOUT = 90mA
4.5
IOUT = 120mA
4.0
3.5
3.0
2.5
2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
VIN [V]
V_IN voltage and LED forward-direction voltage drop limit value
NIPPON PRECISION CIRCUITS INC.—14