Datasheet

TPS61180/1/2
www.ti.com SLVS801E –DECEMBER 2007–REVISED APRIL 2013
Table 1. Recommended Inductor for TPS61180/1/2
L DCR Typ I
sat
Size
(μH) (mΩ) (A) (LXWXH mm)
TOKO
A915AY-4R7M 4.7 38 1.87 5.2x5.2x3.0
A915AY-100M 10 75 1.24 5.2x5.2x3.0
TDK
SLF6028T-4R7M1R6 4.7 28.4 1.6 6.0x6.0x2.8
SLF6028T-100M1R3 10 53.2 1.3 6.0x6.0x2.8
OUTPUT CAPACITOR SELECTION
During PWM brightness dimming, the load transient causes voltage ripple on the output capacitor. Since the
PWM dimming frequency is in the audible frequency range, the ripple can produce audible noises on the output
ceramic capacitor. There are two ways to reduce or eliminate this audible noise. The first option is to select PWM
dimming frequency outside the audible range. This means the dimming frequency needs be to lower than 200Hz
or higher than 30KHz. The potential issue with low dimming frequency is that WLED on/off can become visible
and thus cause a flickering effect on the display. On the other hand, high dimming frequency can compromise
the dimming range since the LED current accuracy and current match are difficult to maintain at low dimming
duty cycle. The TPS61180/1/2 ICs can support minimum 1% dimming duty cycle up to 1KHz dimming frequency.
The second option is to reduce the amount of the output ripple, and therefore minimize the audible noise.
The TPS61180/1/2 ICs adopt a patented technology to limit output ripple even with small output capacitance. In
a typical application, the output ripple is less than 200mV during PWM dimming with 4.7μF output capacitor, and
the audible noise is not noticeable. The devices are designed to be stable with output capacitor down to 1.0μF.
However, the output ripple can increase with lower output capacitor.
Care must be taken when evaluating a ceramic capacitor’s derating due to applied dc voltage, aging and over
frequency. For example, larger form factor capacitors (in 1206 size) have their self resonant frequencies in the
switching frequency range of the TPS61180/1/2. So the effective capacitance is significantly lower. Therefore, it
may be necessary to use small capacitors in parallel instead of one large capacitor.
ISOLATION MOSFET SELECTION
The TPS61180/1/2 ICs provide a gate driver to an external P channel MOSFET which can be turned off during
device shutdown or fault condition. This MOSFET can provide a true shutdown function, and also protect the
battery from output short circuit conditions. The source of the PMOS should be connected to the input, and a pull
up resistor is required between the source and gate of the FET to keep the FET off during IC shutdown. To turn
on the isolation FET, the Fault pin is pulled low, and clamped at 8 V below the V
BAT
pin voltage.
During device shutdown or fault condition, the isolation FET is turned off, and the input voltage is applied on the
isolation MOSFET. During short circuit condition, the catch diode (D2 in typical application circuit) is forward
biased when the isolation FET is turned off. The drain of the isolation FET swings below ground. The voltage
cross the isolation FET can be momentarily greater than the input voltage. Therefore, select 30V PMOS for 24V
maximum input. The on resistor of the FET has large impact on power conversion efficiency since the FET
carries the input voltage. Select a MOSFET with R
ds(on)
less than 100mΩ to limit the power losses.
AUDIBLE NOISE REDUCTION
Ceramic capacitors can produce audible noise if the frequency of its AC voltage ripple is in the audible frequency
range. In TPS61180/1/2 applications, both input and output capacitors are subject to AC voltage ripple during
PWM brightness dimming. The ICs integrate a patented technology to minimize the ripple voltage, and thus
audible noises.
To further reduce the audible noise, one effective way is to use two or three small size capacitors in parallel
instead of one large capacitor. The application circuit in Figure 16 uses two 2.2-μF/25V ceramic capacitors at the
input and two 1-μF/50V ceramic capacitors at the output. All of the capacitors are in 0805 package. Although the
output ripple during PWM dimming is higher than one 4.7μF in a 1206 package, the overall audible noise is
lower.
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