LC101N datasheet(8/15 Pages) SANKEN

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LC101N

LC101N-DSE Rev.1.0

SANKEN ELECTRIC CO.,LTD

Nov.19, 2015

http://www.sanken-ele.co.jp/en/

11. Operational Description

11.1 Basic operation

The current control of the LC101N is quite simple.

The both end voltage of the setup resistor R1 which is

connected between the FB terminal and the LO pin. So

that it becomes constant, the emitter-collector voltage of

the internal PNP-transistor is linear-controlled by the

error amplifier.

Figure 11-1 shows the basic circuit of LC101N.

ICONST

LED

ILED1

LED

ILED2

LED1

LED2

VF1

VF2

VF1×n

VF2×n

Figure 11-1 LC101N basic circuit

LED current, ILED is calculated by Equation (3).

(3)

From Equation (3), the setup resistor, R1 is calculated

by Equation (4).

(4)

The reference voltage, VFB for the constant current

control is 200 mV. The both end voltage of R1 is

controlled to become equal to VFB.

The loss of R1, PR1 is calculated by Equation (5).

(5)

Table 11-1 shows the calculation result of relationship

of ILED, R1, and PR1.

Table 11-1 The loss of R1, PR1

ILED (mA)

R1 (Ω)

PR1 (mW)

100

150

1.33

In addition, the loss of the IC, PD is calculated by

Equation (6).

(6)

where, VIN is the voltage between the IN pin and the

LO pin, and VLED is the voltage between the LED pin

and LO pin.

11.2 Power Supply

The power supply should be use the constant-current

source. If the constant-voltage source is used, the

voltage difference between input and output that does

not require for LED becomes the loss of the IC.

Though it will operate with the constant-voltage

source, if the input-output voltage difference becomes

larger than necessary, and all of surplus will be loss in

this IC

The ability of power supply, PIN is required to satisfy

following equation.

(7)

where, N is number of LED in series,

VF is forward voltage of LED, and

ICONST is the constant current of power supply.

For example, LED current is set to 50 mA in Figure

11-1. The constant current of power supply, ICONST, is set

fewer than the total current value of LED-strings A and

B (100mA), because the drooping characteristic of

constant-current source is considered. In this case, ICONST

is set 95 mA.

If VF = 3V and n = 7, PIN becomes as follows:

11.3 Current Balance Operation

If All LEDs have same VF, and the same current flows

to LED-string A and B, the power loss of IC becomes

minimum because the circuit is controlled by the

constant-current source, ICONST and the internal transistor

is a saturated condition.

However, the complete equilibrium state is impossible

by characteristic variations of VF. If each VF2 of LED

string B is smaller than VF1 of LED string A by 0.1 V.

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