TPS2500, TPS2501

SLVS886C – OCTOBER 2008 – REVISED AUGUST 2010

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Figure 20. Efficiency vs AUX Current

IN Capacitance

Connect the input capacitance from IN to the reference ground plane. (See the Layout Recommendations

section for connecting PGND and GND to the ground plane.) Input capacitance reduces the ac voltage ripple on

the input rail by providing a low-impedance path for the switching current of the boost converter. The TPS2500

does not have a minimum or maximum input capacitance requirement for operation, but a 10-mF, X7R or X5R

ceramic capacitor is recommended for most applications for reasonable input-voltage ripple performance. There

are several scenarios where it is recommended to use additional input capacitance:

•

The output impedance of the upstream power supply is high, or the power supply is located far from the

TPS2500.

•

The TPS2500 is tested in a lab environment with long, inductive cables connected to the input, and transient

voltage spikes could exceed the absolute maximum voltage rating of the device.

•

The device is operating in Eco-mode control scheme near VIN = 1.8 V, where insufficient input capacitance

may cause the input ripple voltage to fall below the minimum 1.75-V (typical) UVLO circuit, causing device

turnoff.

Additionally, it is good engineering practice to use an additional 0.1-mF ceramic decoupling capacitor close to the

IC to prevent unwanted high-frequency noise from coupling into the device.

AUX Capacitance

Connect the boost-converter output capacitance from AUX to the reference ground plane. The AUX capacitance

controls the ripple voltage on the AUX rail and provides a low-impedance path for the switching and

transient-load currents of the boost converter. It also sets the location of the output pole in the control loop of the

boost converter. There are limitations to the minimum and maximum capacitance on AUX. The recommended

minimum capacitance on AUX is a 22-mF, X5R or X7R ceramic capacitor. A 10-V rated ceramic capacitor is

recommended to minimize the capacitance derating loss due to dc bias applied to the capacitor. The low ESR of

the ceramic capacitor minimizes ripple voltage and power dissipation from the large, pulsating currents of the

boost converter and provides adequate phase margin across all recommended operating conditions.

In some applications, it is desirable to add additional AUX capacitance. Additional AUX capacitance reduces

transient undershoot/overshoot voltages due to load steps and reduces AUX ripple in the Eco-mode control

scheme. Adding AUX capacitance changes the control loop, resulting in reduced phase margin, so it is

recommended that no more than 220 mF of additional capacitance be added in parallel to the 22-mF ceramic

capacitor. The combined output capacitance on AUX and USB should not exceed 500 mF.

20

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