Three Terminal Current Controller
© 2014 Exar Corporation 8683DOC rev 1.2 6
Note 1: For ADJ option, the regulating current is determined by an external resistor, REXT, connected between the CS pin and the K pin.
The mode selection function will not change the current ratio of option ADJ. To activate the line regulation function, the chip (U3)
connected in series with the LED string should be set in Mode 0 (MS1 and MS2 connected to pin K). The regulating current will
be:
IPEAK=0.27/REXT
And the maximum regulating current of second step (ex: U2 in Mode 1) should not exceed 80% of the top level (ex: U3 in Mode
0), otherwise the circuit operation might become abnormal when OTP function is activated. It is strongly recommended to set at
75%.
Note 2: The user can add an external resistor REXT between the CS pin and the K pin of U3 (Mode 0, serial connected to the LED string)
to increase the regulating current of option C1, C2, C3, D1, D2 and D3, as shown in below. For U1 ~ U2 (Mode 1 ~ Mode 3,
parallel connected to the LED string), adding an external resistor REXT between the CS pin and the K pin may cause abnormal
operation and chip damage.
For option C1/ C2/ C3, the regulating current variation ΔIPEAK/IPEAK = 6.25/REXT.
For option D1/ D2/ D3, the regulating current variation ΔIPEAK/IPEAK = 3.13/REXT.
LED
iML8683
MS1
MS2
K
CS
NC
A
REXT
Note 3: The Regulating Current Line Regulation is defined as:
For Mode 1~3: ∆LR/=(VAK=0V)-(VAK=5V)
IAK(VAK=5V) , x=1~3
For Mode 0: ∆LR/=(VAK=40V)-(VAK=5V)
IAK(VAK=5V)
Note 4: The CS Pin Voltage Line Regulation is defined as:
For Mode 1~3: ∆
LR/
=(VAK=0V)-(VAK=5V)
(VAK=5V)
For Mode 0: ∆
LR/
=(VAK=40V)-(VAK=5V)
VCS(VAK=5V)
Note 5: Dropout voltage=VAK @ 90% × (IPEAK0 @ VAK=5V)
Note 6: When TJ > TTP, the peak regulating current decreases linearly to around 50% at 175°C.
U3 (Mode 0) regulating current
Without external
resistor
With 100Ω external
resistor