Datasheet

Ipeak + 2 Iload
Vout ) Vf * Vin
L Fs
Ǹ
D + L
Ipeak
Vin
Fs
Iout_max +
Vin
ǒ
Ilim *
I
p
2
Ǔ
h
Vout
I
p
+
1
L
ǒ
1
Vout)Vf*Vin
)
1
Vin
Ǔ
Fs
TPS61080
TPS61081
SLVS644D –FEBRUARY 2006–REVISED APRIL 2013 www.ti.com
FREQUENCY SELECTION
The FSW pin can be connected to either a logic high or logic low to program the switching frequency to1.2MHz
or 600kHz respectively. The 600kHz switching frequency provides better efficiency because of lower switching
losses. This advantage becomes more evident at light load when switching losses dominate overall losses. The
higher switching frequency shrinks external component size and thus the size of power solution. High switching
frequency also improves load transient response since the smaller value inductor takes less time to ramp up and
down current. The other benefits of high switching frequency are lower output ripples and a higher maximum
output current. Overall, it is recommended to use 1.2MHz switching frequency unless light load efficiency is a
major concern.
The FSW pin has internal 800kΩ pull up resistor to the VIN pin. Floating this pin programs the switching
frequency to 1.2MHz.
MAXIMUM and MINIMUM OUTPUT CURRENT
The over-current limit in a boost converter limits the maximum input current and thus maximum input power from
a given input voltage. Maximum output power is less than maximum input power due to power conversion losses.
Therefore, the over-current limit, the input voltage, the output voltage and the conversion efficiency all affect
maximum current output. Since the over-current limit clamps the peak inductor current, the current ripple has to
be subtracted to derive maximum DC current. The current ripple is a function of the switching frequency, the
inductor value and the duty cycle.
(1)
where
Ip = inductor peak to peak ripple
L = inductor value
Vf = power diode forward voltage
Fs = Switching frequency
The following equations take into account of all the above factors for maximum output current calculation.
(2)
where
Ilim = overcurrent limit
η = conversion efficiency
To minimize the variation in the overcurrent limit threshold, the TPS61080/1 uses the VIN and OUT pin voltage to
compensate for the variation caused by the slope compensation. However, the threshold still has some
dependency on the VIN and OUT voltage. Use Figure 6 to Figure 9 to identify the typical over-current limit in
your application, and use 25% tolerance to account for temperature dependency and process variations.
Because of the minimum duty cycle of each power switching cycle of TPS61080/1, the device can lose regulation
at the very light load. Use the following equations to calculate PWM duty cycle under discontinues conduction
mode (DCM).
(3)
Where
Ipeak = inductor peak to peak ripple in DCM
Iload = load current
D = PWM switching duty cycle
12 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: TPS61080 TPS61081