TPS61140_17 Datasheet, PDF (14/26 Pages) Texas Instruments – DUAL OUTPUT BOOST REGULATOR USING SINGLE INDUCTOR

TPS61140
TPS61141
SLVS624B – JANUARY 2006 – REVISED MARCH 2007
APPLICATION INFORMATION
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MAXIMUM OUTPUT CURRENT – PWM CONTROL
The over-current limit in a boost converter limits the maximum input current and thus maximum input power for a
given input voltage. Maximum output power is less than maximum input power due to power conversion losses.
Therefore, the current limit setting, input voltage, output voltage and efficiency can all change maximum current
output. Since current limit clamps peak inductor current, ripple has to be subtracted to derive maximum DC
current. The ripple current is a function of switching frequency, inductor value and duty cycle. The following
equations take into account of all the above factors for maximum output current calculation.
ƪ ǒ Ǔ ƫ ^{Ip +}
1
L
1
Viout)Vf*Vin
)
1
Vin
Fs
(3)
where
Ip = inductor peak to peak ripple
L = inductor value
Vf = power diode forward voltage
Fs = Switching frequency
Viout = boost output voltage. It is equal to the higher of either 330 mV + Vout or 330 mV + voltage drop
across WLED.
ǒ Ǔ Vin
Ilim
*
Ip
2
h
Iout_max +
Viout
(4)
where
Iout_max = Maximum output current of the boost converter
Ilim = overcurrent limit
η = efficiency
To keep a tight range of the overcurrent limit, The TPS61140/1 uses the Vin and Iout pin voltage to compensate
for the overcurrent limit variation caused by the slope compensation. However, the current threshold still has
residual dependency on the Vin and Iout voltage. Use Figure 16 and Figure 17 to identify the typical overcurrent
limit in your application, and use ±25% tolerance to account for temperature dependency and process variations.
The maximum output current can also be limited by the current capability of the LDO and the current sink
circuitry. Both are designed to provide maximum 35 mA current regardless of the current capability of the boost
converter.
MAXIMUM OUTPUT CURRENT – PFM CONTROL
When only voltage output is selected, the boost operates in PFM mode, and the maximum output current can be
calculated as,
Ton +
L
Ilim
Vin
(5)
Toff
+
L
Viout )
Ilim
V_ƒ *
Vin
(6)
h
Iout_max +
Ilim VIN Ton ) Toff
2Viout
Ton ) Toff_min
(7)
T_{off_min} = minimum off time
14
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