IS31LT3117 Datasheet, PDF (11/15 Pages) Integrated Silicon Solution, Inc – 53V, 350MA, 4-CHANNEL CONSTANT CURRENT REGULATOR WITH OTP

IS31LT3117
To address this thermal condition, the IS31LT3117
integrates a 2.5V reference output which can be used
to drive the base of an external BJT. This turns on the
BJT and effectively clamps the voltage across the
IS31LT3117’s output driver to approximately 0.8V. The
power dissipation is then shared between the IC and
the standoff transistor. The VREF pin can source up to
10mA of current to drive 4 external BJT’s, one for each
channel.
OPERATION WITH EXTERNAL BJTS
In most of the applications, the largest power
dissipation will be caused by the current regulator. The
thermal dissipation is proportional to the headroom
voltage (V_VLEDx) and the sink current flowing through it.
When VCC is much higher than the V_LEDS or I_SINKx is
large, the power dissipation of the IS31LT3117 will be
high. This condition may easily trigger the over
temperature protection (OTP). Using external standoff
BJTs can transfer the unwanted thermal power from
the current regulator channel to the BJTs (Figure 15).
Figure 15 IS31LT3117 with external BJTs
With the external BJTs, the voltage across VLEDx to
GND is given by Equation (3):
V_VLEDx  V_REF  V_{beQ 5}  R_x  I _beQx  V_beQx
 V_REF
 V_{beQ 5}
 R_x 
I _SINKx
 1
 V_beQx
(3)
Where V_beQ5 and V_beQx are the base-emitter voltage of
Q₅ and Q_x, I_beQx is the base-emitter current of Q_x.  is
the gain of BJT.
In order to ensure the normal operation, the voltage
across VLEDx should not be lower than the minimum
headroom voltage, minimum V_HD (0.8V). So,
V_REF
 V_{beQ 5}
 R_x 
I _SINKx
 1
 V_beQx
 V_HD
Therefore,
R_x

V_REF
 V_{beQ 5}  V_beQx
I _SINKx
 V_HD
(4)
 1
Integrated Silicon Solution, Inc. – www.issi.com
Rev.0C, 06/19/2014
R₅ can transfer the unwanted thermal power from Q₅ to
itself. Assume the current thought Q₅ is I_Q5,
I_Q5

X
4


1
I _SINKx
 1
(5)
The power on R₅ can be given by Equation (6):
P_R5  R₅  I_Q52
(6)
The power on Q₅ can be given by Equation (7):
    P_Q5  V_CC  V_REF  V_beQ5  R₅  I_Q5  I _Q5 (7)
An appropriate value of R₅ should be chosen to ensure
the power dissipation on Q₅ won’t exceed the power
rating of Q₅. If the sum of total power of P_R5 and P_Q5 is
low enough, R₅ can be shorted and all power
dissipates on Q₅.
The power on Q_x can be calculated by Equation (8):
  P_Qx  V_CC  V_LEDS  V_VLEDx  I _SINKx
(8)
An appropriate value of R_x should be chosen to ensure
the power dissipation on Q_x won’t exceed the power
rating of Q_x.
All of these BJTs should be set to operate in the linear
region to ensure normal operation.
For example, assume I_SINKx =350mA, V_CC=12V, V_LEDS
of three LEDs is 9.6V, the minimum  of the selected
BJT is 200, the maximum base-emitter voltage of Q₅
and Q_x are all 0.7V, The minimum VREF pin output
voltage is 2.4V, The V_be of BJT is approximately 0.7V.
R_x can be calculated from Equation (4):
R_x
 ^VREF
 V_{beQ 5}  V_beQx
I _SINKx
 V_HD
 1

2.4

0.7  0.7
0.35

0.8
 115
200  1
By Equation (5),
I_Q5

4

X
1
I _SINKx
 1

4
0.35
200  1

7mA
Therefore,
  P_S  P_Q5  P_R5  V_CC  (V_REF  V_beQ5 )  I_Q5
 12  2.4  0.7 0.007  0.0721W
The P_S is pretty low. So R₅ can be eliminated.
And,
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