TPS62619, TPS62612

TPS62615, TPS62616

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SLVS936A – NOVEMBER 2009 – REVISED MARCH 2010

OUTPUT CAPACITOR SELECTION

The advanced fast-response voltage mode control scheme of the TPS6261x allows the use of tiny ceramic

capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are

recommended. For best performance, the device should be operated with a minimum effective output

capacitance of 1.6mF. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric

capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequencies.

At nominal load current, the device operates in PWM mode and the overall output voltage ripple is the sum of the

voltage step caused by the output capacitor ESL and the ripple current flowing through the output capacitor

impedance.

At light loads, the output capacitor limits the output ripple voltage and provides holdup during large load

transitions. A 4.7mF capacitor typically provides sufficient bulk capacitance to stabilize the output during large

The output voltage ripple during PFM mode operation can be kept very small. The PFM pulse is time controlled,

which allows to modify the charge transferred to the output capacitor by the value of the inductor. The resulting

PFM output voltage ripple and PFM frequency depend in first order on the size of the output capacitor and the

inductor value. The PFM frequency decreases with smaller inductor values and increases with larger once.

Increasing the output capacitor value and the effective inductance will minimize the output ripple voltage.

INPUT CAPACITOR SELECTION

Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is

required to prevent large voltage transients that can cause misbehavior of the device or interferences with other

circuits in the system. For most applications, a 2.2-mF capacitor is sufficient. If the application exhibits a noisy or

erratic switching frequency, the remedy will probably be found by experimenting with the value of the input

capacitor.

Take care when using only ceramic input capacitors. When a ceramic capacitor is used at the input and the

power is being supplied through long wires, such as from a wall adapter, a load step at the output can induce

ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop instability or could even

damage the part. Additional "bulk" capacitance (electrolytic or tantalum) should in this circumstance be placed

CHECKING LOOP STABILITY

The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signals:

•

Switching node, SW

•

•

These are the basic signals that need to be measured when evaluating a switching converter. When the

switching waveform shows large duty cycle jitter or the output voltage or inductor current shows oscillations, the

regulation loop may be unstable. This is often a result of board layout and/or L-C combination.

As a next step in the evaluation of the regulation loop, the load transient response is tested. The time between

the application of the load transient and the turn on of the P-channel MOSFET, the output capacitor must supply

the device operates in PWM mode.

converter’s stability. Without any ringing, the loop has usually more than 45° of phase margin.

Because the damping factor of the circuitry is directly related to several resistive parameters (e.g., MOSFET

load current range, and temperature range.

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Product Folder Link(s): TPS62619 TPS62612 TPS62615 TPS62616