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| TPS92601-Q1_15 Datasheet, PDF (29/45 Pages) Texas Instruments – TPS9260x-Q1 Single- and Dual-Channel Automotive Headlight LED Driver | |||
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TPS92601-Q1, TPS92602-Q1, TPS92601A-Q1, TPS92602A-Q1
SLUSBP5D â MARCH 2014 â REVISED JANUARY 2015
9.2.2.2.1 Switching Frequency
The RT pin resistor sets the switching frequency of the TPS92602-Q1 device to 600 kHz. Use Equation 2 to
calculate the required value for R17. The calculated value is 20.83 kΩ. Use the nearest standard value of 20 kΩ.
9.2.2.2.2 Maximum Output-Current Set Point
The output constant of the TPS92602-Q1 device is adjustable by using the external current-shunt resistor. In the
application circuit of Figure 22, R5 is the channel 1 current-shunt resistor, and R16 is the channel-2 current shunt
resistor. Equation 46 and Equation 47 calculate the resistors that determine maximum output current.
R(sense) = VSPSN_Diff / I(setting)
R5 = R16 = 150 mV / 1 A = 0.15 Ω
(46)
(47)
9.2.2.2.3 Output Overvoltage-Protection Set Point
The output overvoltage protection threshold of the TPS92602-Q1 device is externally adjustable using a resistor
divider network. In the application circuit of Figure 22, this divider network comprises of R1 and R3 for channel1
and R2 and R4 for channel2. The following equation gives the relationship of the overvoltage-protection
threshold (V(OVPT)) to the resistor divider.
R1 / R3 = R2 / R4 = (V(OVPT) â V(VFB)) / V(VFB)
(48)
The load is four white LEDs, the forward voltage is about 13.2 V, maximum V(VIN) is 16 V, so the maximum
output is 13.2 + 16 = 29.2 V, which is close to 30 V. Allowing 20% margin for overvoltage protection, V(OVPT) is:
V(OVPT) = 30 à 1.2 = 36 V. So R1 / R3 = R2 / R4 = (36 â 2.2) / 2.2 = 15.36. Select R3 = R4 = 30 kΩ; then R1 =
R2 = 460 kΩ. Use the nearest standard value of 464 kΩ.
9.2.2.2.4 Duty Cycle Estimation
Estimate the duty cycle of the main switching MOSFET using Equation 49 and Equation 50.
D (MIN)
»
V(LED) + V(FD)
V(LED) + V(MAX) + V(FD)
=
13.2 V + 0.5 V
30 V + 16 V + 0.5 V
=
46.1%
where
⢠D is the duty cycle in these and all following equations
(49)
D (MAX )
»
V(LED) + V(FD)
V(LED) + V(MIN) + V(FD)
13.2 V + 0.5 V
=
13.2 V + 6 V + 0.5 V
=
69.5%
(50)
Using an estimated forward drop of 0.5 V for a Schottky rectifier diode, the approximate duty cycle is 46.1%
(minimum) to 69.5% (maximum).
9.2.2.2.5 Inductor Selection
The peak-to-peak ripple is limited to 30% of the maximum output current.
I(Lrip- max)
= 0.3 ´ I(OUT-max)
1- D(MIN)
=
0.3
´
1
-
1
0.461
=
0.556
A
(51)
Estimate the minimum inductor size using Equation 52
L (MIN)
>>
V(IN- max)
I(Lrip-max )
´
D (MIN)
´
1
f(SW
)
=
16 V ´ 0.475 ´ 1
0.571 A
600 kHz
= 22.1 mH
(52)
Select the nearest higher standard inductor value of 22 µH. Estimate the ripple current using Equation 53.
I(RIPPLE)
»
V(IN)
L
´
D (MIN)
´
1
f(SW
)
=
16 V ´ 0.461´ 1
22 mH
600 kHz
= 0.559
A
(53)
I(RIPPLE-Vin min)
»
V(IN)
L
´
D (MIN)
´
1
f(SW )
=
6 V ´ 0.695 ´ 1
22 mH
600 kHz
= 0.316
A
(54)
The worst-case peak-to-peak ripple current occurs at 46.1% duty cycle and is estimated as 0.559 A. Equation 55
estimates the worst-case rms current through the inductor.
Copyright © 2014â2015, Texas Instruments Incorporated
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Product Folder Links: TPS92601-Q1 TPS92602-Q1 TPS92601A-Q1 TPS92602A-Q1
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