Texas Instruments TPS61181(82/81A)-259 Evaluation Board TPS61181(82/81A)EVM-259 TPS61181AEVM-259 TPS61181AEVM-259 Datenbogen
Produktcode
TPS61181AEVM-259
SLVSAN6A
–
FEBRUARY 2011
–
REVISED MARCH 2011
Table 1. Recommended Inductor for TPS61181A
L
DCR Typ
I
sat
Size
(
μ
H)
(m
Ω
)
(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 a very 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 second option is to reduce the amount of the output ripple, and therefore minimize the
audible noise.
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 a very 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 second option is to reduce the amount of the output ripple, and therefore minimize the
audible noise.
The TPS61181A adopts 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 a 4.7
typical application, the output ripple is less than 200mV during PWM dimming with a 4.7
μ
F output capacitor, and
the audible noise is not noticeable. The devices are designed to be stable with output capacitor down to 1.0
μ
F.
However, the output ripple will 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 TPS61181A. So the effective capacitance is significantly lower. Therefore, it
may be necessary to use small capacitors in parallel instead of one large capacitor.
switching frequency range of the TPS61181A. So the effective capacitance is significantly lower. Therefore, it
may be necessary to use small capacitors in parallel instead of one large capacitor.
AUDIBLE NOISE REDUCTION
Ceramic capacitors can produce audible noise if the frequency of its AC voltage ripple is in the audible frequency
range. In TPS61181A 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.
range. In TPS61181A 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
instead of one large capacitor. The application circuit in
uses two 2.2-
μ
F/25V ceramic capacitors at the
input and two 1-
μ
F/50V ceramic capacitors at the output. All of the capacitors are in 0805 package. Although the
output ripple during PWM dimming is higher than with one 4.7
μ
F in a 1206 package, the overall audible noise is
lower.
In addition, connecting a 10-nF/50V ceramic capacitor between the V
O
pin and IFB1 pin can further reduce the
output AC ripple during the PWM dimming. Since this capacitor is subject to large AC ripple, choose a small
package such as 0402 to prevent it from producing noise.
package such as 0402 to prevent it from producing noise.
ISOLATION MOSFET SELECTION
The TPS61181A provides 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
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
Copyright
©
2011, Texas Instruments Incorporated
15
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