Datasheet

AUX TOTAL
IN
IN
V I
5.1 V 1 A
I 2.1 A
V 2.7 V 0.9
´
´
= = =
´ h ´
Time
Di
I
L
I
IN
I
L_pk
IN
i 0.3 I 0.3 2.1 A 630 mAD = ´ = ´ =
L_pk IN
i
I I 2.42 A 3 A
2
D
= + = <
( )
( )
( )
( )
AUX IN IN SYNC L
on
on off AUX IN SYNC PWM
V V I R R
t
D
t t V I R R
5.1 V 2.7 V 2.1 A 0.1.Ω 0.07.Ω
0.54
5.1 V 2.1 A 0.1.Ω 0.1.Ω
é ù
- + ´ +
= =
ê ú
+ + ´ -
ê ú
ë û
é ù
- + ´ +
= =
ê ú
+ ´ -
ê ú
ë û
IN
6
V D
2.7 V 0.54
L 2.31.μH
f i
1 10 Hz 0.63 A
´
´
= = =
´ D
´ ´
TPS2500, TPS2501
SLVS886C –OCTOBER 2008–REVISED AUGUST 2010 www.ti.com
AUX Voltage
The AUX voltage of the TPS2500 is internally fixed at 5.1 V.
Determine Maximum Total Current (I
AUX
+ I
USB
)
Using Figure 12, the maximum total current at V
IN
= 2.7 V is 1 A using the conservative line. The design
requirements are met for this application.
Power Inductor
The inductor ripple current, Δi, should be at least 20% of the average inductor current to avoid erratic operation
of the peak-current-mode PWM controller. Assume an inductor ripple current, Δi, which is 30% of the average
inductor current and a power-converter efficiency, h, of 90%. Using the minimum input voltage, the average
inductor current at V
IN
= 2.7 V is:
Figure 23. Waveform of Current in Boost Inductor
The corresponding inductor ripple current is:
Verify that the peak inductor current is less than the 3-A peak switch current:
The following equation estimates the duty cycle of the low-side PWM MOSFET:
where R
PWM
is the low-side control MOSFET on-resistance, R
SYNC
is the high-side synchronous MOSFET
on-resistance, and R
L
is an estimate of the inductor dc resistance.
The following equation calculates the recommended inductance for this design.
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