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TPS92641PWPT Datasheet(PDF) 15 Page - Texas Instruments

No. de Pieza. TPS92641PWPT
Descripción  Synchronous Buck Controllers for Precision Dimming LED Drivers
Descarga  37 Pages
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Fabricante  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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TPS92641PWPT Datasheet(HTML) 15 Page - Texas Instruments

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LED
DIM
LED
_
DIM
I
D
I
u
tOFF
iLED (t)
ILED-MAX
t
IDIM-LED
0
JDDIM x TDIMJ
JTDIMJ
ILED
¸¸
¹
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u
P
2
UDIM
2
UDIM
1
UDIM
3
UDIM
1
UDIM
HYS
R
R
R
R
R
A
21
V
1
UDIM
HYS
R
A
21
V
u
P
¸
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u
2
UDIM
2
UDIM
1
UDIM
ON
_
TURN
R
R
R
V
276
.
1
V
»
¼
º
«
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u
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OUT
CS
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TPS92640, TPS92641
www.ti.com
SNVS902A – OCTOBER 2012 – REVISED OCTOBER 2015
Feature Description (continued)
To conclude, an external resistor (ROFF) from VOUT to CS pin is required for true zero application, where ROFF
should be:
(7)
7.3.5 Undervoltage Lockout (UVLO)
The UDIM pin of the TPS92640 and TPS92641 devices is a dual function input that features an accurate 1.276-V
threshold with programmable hysteresis. This pin functions as both the PWM dimming input of the LEDs and as
an input UVLO with built-in hysteresis. When the pin voltage rises and exceeds the 1.276-V threshold, 21 µA
(typical) of current is driven out of the UDIM pin into the resistor divider (RUDIM1, RUDIM2) providing programmable
hysteresis. The UVLO turnon threshold, VTURN-ON, is defined using Equation 8.
(8)
Once the input voltage is above VTURN_ON, the current source is active and the UVLO hysteresis is determined by
Equation 9.
(9)
When using the UDIM pin for UVLO and PWM dimming concurrently, the UVLO circuit can have an extra resistor
(RUDIM3) to set the hysteresis. This allows the standard resistor divider to have smaller values minimizing delays
that can incur with additional external PWM dimming circuitry. In general, at least 3 V of hysteresis is preferable
when PWM dimming if operating near the UVLO threshold. Under these conditions, the UVLO hysteresis is
defined using Equation 10.
(10)
7.3.6 PWM Dimming Using the UDIM Pin
The UDIM pin can be driven with a PWM signal, which controls the synchronous NFET operation. The brightness
of the LEDs can be varied by modulating the duty cycle (DDIM) of this signal using a Schottky diode with anode
connected to UDIM pin, as shown in Figure 13.
Figure 19. LED Current During UDIM Pin PWM Dimming
Figure 19 shows the LED current waveform during PWM dimming where duty cycle (DDIM) is the percentage of
the dimming period (TDIM) that the synchronous NFETs are switching. For the remainder of TDIM, the NFETs are
disabled. The resulting dimmed LED current (IDIM_LED) is:
(11)
Copyright © 2012–2015, Texas Instruments Incorporated
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Product Folder Links: TPS92640 TPS92641


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