Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB データシート
製品コード
LM3448-EDSNEV/NOPB
R3
D1
Q1
R4
V+
C7
C8
R22
D8
LM3448
6
VCC
R17
Q2
D9
R12
D11
D10
C4
T1
SW
VLED+
VLED-
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
Figure 31. V
CC
Auxiliary Winding Bias Circuit
COFF CURRENT SOURCE CIRCUITS
There are a few different current source circuits that can be used for establishing the LM3448 constant-off time
control as shown in
control as shown in
(a) shows the simplest current source circuit. Capacitor C
OFF
will be charged with a constant current
from V
CC
through resistor R
OFF
.
If there is large noise or ripple on the VCC pin, then the previously described circuit will fluctuate and the off-time
will not be constant. The circuit of
will not be constant. The circuit of
(b) addresses this by using a zener diode D1 across R
OFF
which
establishes a stable voltage reference for the current source with inherent V
CC
ripple rejection.
LED loads can exhibit voltage drift due to self-heating or external thermal conditions. A change in the LED stack
voltage will result in the LED current to drift as well.
voltage will result in the LED current to drift as well.
(c) addresses this issue by having the COFF
current source referenced to the LED stack voltage using Q1 and R
OFF
and thereby compensating for LED
voltage drift. Another benefit is that the number of series LEDs in the LED string can be changed while still
maintaining the same output drive current.
maintaining the same output drive current.
Copyright © 2011–2013, Texas Instruments Incorporated
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