LM3429_14 Datasheet, PDF (12/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
Boost and Buck-boost
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f_SW = _{RT x CT}
(6)
For all topologies, the C_T capacitor is recommended to be 1 nF and should be located very close to the LM3429.
AVERAGE LED CURRENT
The LM3429 uses an external current sense resistor (R_SNS) placed in series with the LED load to convert the
LED current (I_LED) into a voltage (V_SNS) as shown in Figure 5. The HSP and HSN pins are the inputs to the high-
side sense amplifier which are forced to be equal potential (V_HSP=V_HSN) through negative feedback. Because of
this, the V_SNS voltage is forced across R_HSP to generate the signal current (I_CSH) which flows out of the CSH pin
and through the R_CSH resistor. The error amplifier will regulate the CSH pin to 1.24V, therefore I_CSH can be
calculated:
I_CSH
=
V_SNS
R_HSP
(7)
This means V_SNS will be regulated as follows:
V_SNS
=
1.24V
x
R_HSP
R_CSH
(8)
I_LED can then be calculated:
I_LED =
V_SNS
R_SNS
=
1.24V
R_SNS
x
R_HSP
R_CSH
(9)
The selection of the three resistors (R_SNS, R_CSH, and R_HSP) is not arbitrary. For matching and noise performance,
the suggested signal current I_CSH is approximately 100 µA. This current does not flow in the LEDs and will not
affect either the off state LED current or the regulated LED current. I_CSH can be above or below this value, but
the high-side amplifier offset characteristics may be affected slightly. In addition, to minimize the effect of the
high-side amplifier voltage offset on LED current accuracy, the minimum V_SNS is suggested to be 50 mV. Finally,
a resistor (R_HSN = R_HSP) should be placed in series with the HSN pin to cancel out the effects of the input bias
current (~10 µA) of both inputs of the high-side sense amplifier. Note that he CSH pin can also be used as a low-
side current sense input regulated to the 1.24V. The high-side sense amplifier is disabled if HSP and HSN are
tied to GND.
I_LED
V_SNS RSNS
R_HSP
R_HSN
LM3429
High-Side
HSP ^{Sense Amplifier}
HSN
I_CSH
R_CSH
C_CMP
CSH
COMP
1.24V
Error Amplifier
To PWM
Comparator
Figure 5. LED Current Sense Circuitry
ANALOG DIMMING
The CSH pin can be used to analog dim the LED current by adjusting the current sense voltage (V_SNS). There
are several different methods to adjust V_SNS using the CSH pin:
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