I

1 *

Vout

Vin

L

ƒ

1

2

3

TLV62065

SLVSAC4 – NOVEMBER 2010

www.ti.com

In case additional ceramic capacitors in the supplied system are connected to the output of the DC/DC converter,

range. In this case a loop stability analysis must be performed as described later.

At nominal load current, the device operates in PWM mode and the RMS ripple current is calculated as:

(3)

Input Capacitor Selection

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

required for best input voltage filtering and minimizing the interference with other circuits caused by high input

voltage spikes. For most applications a 10µF ceramic capacitor is recommended. The input capacitor can be

increased without any limit for better input voltage filtering.

Take care when using only small 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 or VIN step on

instability or could even damage the part by exceeding the maximum ratings.

Table 3. List of Capacitors

CAPACITANCE

TYPE

SUPPLIER

10mF

GRM188R60J106M

0603: 1.6 x 0.8 x 0.8

Murata

22mF

GRM188R60G226M

0603: 1.6 x 0.8 x 0.8

Murata

22µF

CL10A226MQ8NRNC

0603: 1.6 x 0.8 x 0.8

Samsung

10µF

CL10A106MQ8NRNC

0603: 1.6 x 0.8 x 0.8

Samsung

CHECKING LOOP STABILITY

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

•

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 wrong L-C output filter

combinations. 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

generating a feedback error signal used by the regulator to return VOUT to its steady-state value. The results are

most easily interpreted when the device operates in PWM mode at medium to high load currents.

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

18

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