Datasheet

I
FB
+ K
ISET
V
ISET
R
ISET
TPS61180/1/2
SLVS801E –DECEMBER 2007–REVISED APRIL 2013 www.ti.com
CURRENT PROGRAM AND PWM DIMMING
The six current sink regulators can each provide maximum 25mA. The IFB current must be programmed to
highest WLED current expected using the ISET pin resistor and the following Equation 1.
(1)
Where
K
ISET
= Current multiple (1000 typical)
V
ISET
= ISET pin voltage (1.229 V typical)
R
ISET
= ISET pin resistor
The TPS61180/1/2 ICs have built-in precise current sink regulator. The current matching among 6 current sinks
is below 2.5%. This means the differential value between the maximum and minimum current of the six current
sinks divided by the average current of the six is less than 2.5%.
The WLED brightness is controlled by the PWM signal on the DCTRL pin. The frequency and duty cycle of the
DCTRL signal is replicated on the IFB pin current. Keep the dimming frequency in the range of 100Hz to 1kHz to
avoid screen flickering and maintain dimming linearity. Screen flickering may occur if the dimming frequency is
below the range. The minimum achievable duty cycle increases with the dimming frequency. For example, while
a 0.1% dimming duty cycle, giving a 1000:1 dimming range, is achievable at 100 Hz dimming frequency, only 1%
duty cycle, giving a 100:1 dimming range, is achievable with a 1 KHz dimming frequency, and 5% dimming duty
cycle is achievable with 5KHz dimming frequency. The device could work at high dimming frequency like 20 KHz,
but only 15% duty cycle could be achievable. The TPS61180/1/2 ICs are designed to minimize the AC ripple on
the output capacitor during PWM dimming. Careful passive component selection is also critical to minimize AC
ripple on the output capacitor. See APPLICATION INFORMATION for more information.
ENABLE AND START UP
A logic high signal on the EN pin turns on the IC. For the TPS61181/2 ICs, taking EN high turns on the internal
LDO linear regulator which provides supply IC current. For all devices, an internal resistor R
start
(start up charging
resistor) is connected between the V
BAT
pin and V
O
pin to charge the output capacitor toward Vin. The Fault pin
outputs high during this time, and thus the external isolation PFET is turned off. Once the V
O
pin voltage is within
2 V (isolation FET start up threshold) of the V
BAT
pin voltage, R
start
is open, and the Fault pin pulls down the gate
of the PFET and connects the V
BAT
voltage to the boost regulator. This operation is to prevent the in-rush current
due to charging the output capacitor.
Once the isolation FET is turned on, the IC starts the PWM switching to raise the output voltage above V
BAT
.
Soft-start is implemented by gradually ramping up the reference voltage of the error amplifier to prevent voltage
over-shoot and in-rush current. See the start-up waveform of a typical example, Figure 15.
Pulling the EN pin low for 32ms (typical) shuts down the IC, resulting in the IC consuming less than 50μA in the
shutdown mode.
OVER-CURRENT, OVER-VOLTAGE AND SHORT-CIRCUIT PROTECTION
The TPS61180/1/2 family has pulse by pulse over-current limit of 1.5A (min). The PWM switch turns off when the
inductor current reaches this current threshold. The PWM switch remains off until the beginning of the next
switching cycle. This protects the IC and external component under over-load conditions. When there is
sustained over-current condition for more than 16ms ( under 100% dimming duty cycle), the IC turns off and
requires PER or the EN pin toggling to restart.
Under severe over-load and/or short circuit conditions, the V
O
pin can be pulled below the input (V
BAT
pin). Under
this condition, the current can follow directly from input to output through the inductor and Schottky diode.
Turning off the PWM switch alone does not limit current anymore. In this case, the TPS61180/1/2 ICs detect the
output voltage is 1V (short circuit detection threshold) below the input voltage, turns off the isolation FET, and
shuts down the IC. The IC restarts after input power-on reset (V
BAT
POR) or EN pin logic toggling.
During the IC start up, if there is short circuit condition on the boost converter output, the output capacitor will not
be charged to within 2V of V
BAT
through R
start
. After 32ms (short circuit detection delay during start up), the IC
shuts down and does not restart until there is V
BAT
POR or EN pin toggling. The isolation FET is never turned on
under the condition.
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