SC172 datasheet(19/27 Pages) SEMTECH

19 / 27 page

Applications Information (continued)

The use of 1% feedback resistors may result in up to an

additional 1% error. If tighter DC accuracy is required, re-

sistors with lower tolerances should be used.

The output inductor value may change with current. This

will change the output ripple and therefore will have a

minor effect on the DC output voltage. The output ESR

also affects the output ripple and thus has a minor effect

on the DC output voltage.

Switching Frequency Variation

The switching frequency will vary depending on line and

load conditions. The line variations are a result of fixed

propagation delays in the on-time one-shot, as well as

unavoidable delays in the power FET switching. As V

increases, these factors make the actual DH on-time

slightly longer than the ideal on-time. The net effect is

that frequency tends to fall slightly with increasing input

voltage.

The switching frequency also varies with load current

as a result of the power losses in the MOSFETs and the

inductor. For a conventional PWM constant-frequency

converter, as load increases the duty cycle also increases

slightly to compensate for IR and switching losses in the

MOSFETs and inductor. A adaptive on-time converter

must also compensate for the same losses by increasing

the effective duty cycle (more time is spent drawing en-

ergy from V

IN as losses increase). The on-time is essential-

ly constant for a given V

OUT and VIN combination, to offset

the losses the off-time will tend to reduce slightly as load

increases. The net effect is that switching frequency in-

creases slightly with increasing load.

Switching Node Voltage Spike

Due to parasitic board inductance, the transient LX pin

voltage at the point of measurement may appear larger

than that which exists on silicon. With an input multilay-

er ceramic capacitor of 10uF placed less than 3mm away

from the PVIN pin, the device is designed and guaranteed

to tolerate the short transient voltages, of maximum

20ns duration, that will appear on the LX pin due to the

deadtime diode conduction, as long as the transient volt-

age on PVIN is less than 6.0V. The time duration of the

transient LX pin voltage is measured on the voltage por-

tion which is either over 6.0V for positive voltage spike

or under -1V for negative voltage spike. The LX voltage

is measured from the LX pin to the PGND pin by using a

probing loop which is as short as possible to minimize or

eliminate the switching noise pick up.

SC172

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