Notes on Constant Current Control Settings

The LB1939T constant current control circuit has the

structure shown in the figure at the right. The voltage input

to the VC pin is resistor divided internally (by 70 k

Ω and

20 k

Ω resistors) to 1/4.5 and input to the plus (+) input of

the constant current control amplifier as reference.

The minus (–) input of this constant current control

amplifier is connected, through the wire bond resistor Rb

(= 0.1

Ω), to the RFG pin. The constant current control

circuit operates by comparing the voltage generated by the

external current detection resistor connected to the RFG

pin and the reference voltage mentioned above.

Note that the voltage at VA will be that given by the

following formula since the bias current Ib (= 1.5 µA)

flows from the constant current control amplifier plus (+)

input during constant current control operation.

VA = VC/4.5 + Ib

× 20 kΩ

= VC/4.5 + 0.03

Therefore, the logical expression for setting the constant

current Iout is as follows.

Iout = VA/(RFB + Rb)

= (VC/4.5 + 0.03) / (RFB + Rb) ......(1)

No. 6194-6/7

LB1939T

Application Circuit Example 3

ENA1

ENA2

IN1

IN2

S-GND

SVDD

Vref

VC1

VC2 RFG1 RFG2

OUT1

OUT2

VDD

CPU

VS2

VS1

OUT3

OUT4

Channel 1 operates in constant voltage control mode: OUT outputs V

Channel 2 operates in constant current control mode: The RFG voltage is controlled so that Vref/4.5 = 0.2 V.

* : FC1 is left open.

FC2

6 k

Ω

3 k

Ω

LB1939T

VS

RL

RFG

OUT1

OUT2

Rb = 0.1

Ω

RFB

Iout

VC

Constant current

control amplifier

Pad

VA

Ib = 1.5

µA

70 k

Ω

20 k

Ω

A13864