Data Sheet

LT7153SP

analog.com

Rev 0

21 of 31

Using Ceramic Input and Output Capacitors

Higher values, 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.

Minimum Off-Time and Minimum On-Time Considerations

The minimum off-time, tOFF(MIN), is the smallest amount of time that the LT7153SP is capable of turning on the bottom

power MOSFET, tripping the current comparator and turning the power MOSFET back off. This time is generally about

30ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is

reached due to a dropping input voltage, for example, then the output drops out of regulation. The minimum input

voltage to avoid dropout is:

VIN(MIN) = VOUT •

tON + tOFF(MIN)

tON

Conversely, the minimum on-time is the smallest duration of time in which the top power MOSFET can be in its ON

state. This time is typically 15ns. In continuous mode operation, the minimum on-time limit imposes a minimum

duty cycle of

DC(MIN) = fSW • tON(MIN)

where tON(MIN) is the minimum on-time. Reducing the operating frequency alleviates the minimum duty cycle

constraint.

In the rare cases where the minimum duty cycle is surpassed, the output voltage remains in regulation and the

switching frequency decreases from its programmed value. This is an acceptable result in many applications. So, this

constraint may not be of critical importance in most cases. High switching frequencies may be used in the design

without any fear of output overvoltage. As the sections on inductors and capacitor selection show, high switching

frequencies allow the use of smaller board components, thus reducing the size of the application circuit.