Data Sheet

LT7200S

analog.com

Rev 0

19 of 35

Selecting the Output Capacitor (C

OUT)

The selection of COUT is determined by the effective series resistance (ESR) required to minimize voltage ripple and

load step transients, as well as the amount of bulk capacitance necessary to ensure the control loop is stable. Loop

stability can be checked by viewing the load transient response. The output ripple, ΔVOUT, is determined by:

∆VOUT < ∆IL(

1

8 x f x COUT

+ ESR)

The output ripple is highest at maximum input voltage since ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic, and ceramic capacitors are all available in surface mount

packages. Special polymer capacitors are very low ESR but have lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density, but it is important to only use types that are surge tested

for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used

in cost-sensitive applications provided consideration is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteristics and small footprints.

Since the ESR of a ceramic capacitor is so low, it is more useful to choose the output capacitor value to fulfill a charge

storage requirement. During a load step, the output capacitor must instantaneously supply the current to support

the load until the feedback loop raises the switch current enough to support the load. The time required for the

feedback loop to respond is dependent on the compensation and the output capacitor size. Typically, five cycles are

required to respond to a load step, but only in the first cycle does the output voltage drop linearly. The output droop,

VDROOP, is usually about three times the linear drop of the first cycle. Thus, a good place to start with the output

capacitor value is approximately

COUT = 3

∆IOUT

f0 x VDROOP

Using Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple

current, high voltage rating, and low ESR make them ideal for switching regulator applications. However, due to the

self-resonant and high Q characteristics of some types of ceramic capacitors, take care when these capacitors are

used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall

adapter through long wires, a load step at the output can induce ringing at the VIN input. At best, this ringing can

couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires

can potentially cause a voltage spike at VIN large enough to damage the part. For a more detailed discussion, refer to

Application Note 88.

When choosing the input and output ceramic capacitors, choose the X5R and X7R dielectric formulations. These

dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size.

Choose X8R for 150°C applications.

More capacitance may be required depending on the duty cycle and load step requirements. In most applications,

the input capacitor is merely required to supply high frequency bypassing, since the impedance to the supply is very

low. A 47μF ceramic capacitor is usually enough for these conditions. Place this input capacitor as close to the PVIN

and GND pins as possible.