L_pk

IN

i

I

I

2.42 A

2

D

=

+

=

(

)

2

2

2

2

L_RMS

IN

i

0.63 A

I

I

2.1 A

2.11 A

2

3

2

3

æ

ö

æ

ö

D

=

+

=

+

=

ç

÷

ç

÷

×

×

è

ø

è

ø

TOTAL

AUX

AUX

D

I

C

f

V

´

=

´ D

o_min

6

0.54 1 A

C

10.8. F

1 10 Hz 50 mV

´

=

=

´

´

IN

6

IN

i

630 mA

C

5.25. F

8 f

V

8 1 10 Hz 15 mV

D

=

=

=

´ ´ D

´ ´

´

TPS2500, TPS2501

www.ti.com

SLVS886C – OCTOBER 2008 – REVISED AUGUST 2010

The peak inductor current is:

The rms inductor current is:

Select a Coilcraft LPS4018-222ML inductor. This 2.2-mH inductor has a saturation current rating of 2.7 A and an

rms current rating of 2.3 A. See the Component Recommendations section for specific additional information.

Output AUX Capacitor Selection

The AUX output capacitor, CAUX, discharges during the PWM MOSFET on-time, resulting in an output ripple

voltage of

ΔVAUX. ΔVAUX is largest at maximum load current.

Ceramic capacitors exhibit a dc bias effect, whereby the capacitance falls with increasing bias voltage. The effect

is worse for capacitors in smaller case sizes and lower voltage ratings. X5R and X7R capacitors exhibit less dc

bias effect than Y5V and Z5U capacitors.

Select a TDK C3225X5R1A226M 22-mF, 10-V X5R ceramic capacitor to allow for a 50% drop in capacitance due

to the dc bias effect. See the Component Recommendations section for specific additional information.

Output USB Capacitor Selection

The USB output capacitor provides energy during a load step on the USB output. The TPS2500 does not require

a USB output capacitor, but many USB applications require that downstream-facing ports be bypassed with a

minimum of 120-mF, low-ESR capacitance.

Select a Panasonic EEVFK1A151P 150-mF, 10-V capacitor.

Input Capacitor Selection

The ripple current through the input filter capacitor is equal to the ripple current through the inductor. If the ESL

and ESR of the input filter capacitor are ignored, then the required input filter capacitance is:

Select a TDK C2012X5R1A106K 10-mF, 10-V, X5R, size 805 ceramic capacitor. The capacitance drops 20% at

3.3-V bias, resulting in an effective capacitance of 8 mF.

An additional 0.1-mF ceramic capacitor should be placed locally from IN to GND to prevent noise from coupling

into the device if the input capacitor cannot be located physically near to the device.

In applications where long, inductive cables connect the input power supply to the device, additional bulk input

capacitance may be necessary to minimize voltage overshoot. See the Component Recommendations section

for specific additional information.

Current-Limit Threshold Resistor RILIM

The current-limit threshold IOS of the power switch is externally adjustable by selecting the RILIM resistor. To

eliminate the possibility of false tripping, RILIM should be selected so that the minimum tolerance of the

current-limit threshold is greater than the maximum specified USB load, IUSB. For design margin, an additional

10% (50 mA) buffer is added above the maximum continuous load current and minimum current-limit threshold,

which sets the minimum desired current-limit threshold at 550 mA.

Copyright © 2008–2010, Texas Instruments Incorporated

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