Applications Information: continued

8

Transient Response

vides unprecedented transient response to changes in input

voltage or output current. Pulse by pulse adjustment of

duty cycle is provided to quickly ramp the inductor current

to the required level. Since the inductor current cannot be

changed instantaneously, regulation is maintained by the

output capacitor(s) during the time required to slew the

inductor current.

Overall load transient response is further improved

through a feature called “adaptive voltage positioning”.

This technique pre-positions the output capacitor’s voltage

to reduce total output voltage excursions during changes in

load.

Holding tolerance to 1% allows the error amplifier’s refer-

ence voltage to be targeted +40mV high without compro-

mising DC accuracy. A “droop resistor“, implemented

through a PC board trace, connects the error amplifier’s

carries the output current. With no load, there is no DC

drop across this resistor, producing an output voltage

tracking the error amplifier’s, including the +40mV offset.

When the full load current is delivered, an 80mV drop is

developed across this resistor. This results in output volt-

age being offset -40mV low.

The result of adaptive voltage positioning is that additional

margin is provided for a load transient before reaching the

output voltage specification limits. When load current sud-

denly increases from its minimum level, the output capaci-

tor is pre-positioned +40mV. Conversely, when load cur-

rent suddenly decreases from its maximum level, the out-

put capacitor is pre-positioned -40mV (see Figures 7, 8, and

9). For best transient response, a combination of a number

of high frequency and bulk output capacitors are usually

used.

If the maximum on time is exceeded while responding to a

sudden increase in load current, a normal off time occurs to

prevent saturation of the output inductor.

Figure 7: CS5156 demonstration board response to a 0.5 to 13A load

pulse (output set for 2.8V).

Figure 8: CS5156 demonstration board response to 13A load turn on

trol loop immediately connects the inductor to the input voltage, pro-

viding 100% duty cycle. Regulation is achieved in less than 20µs.

Trace 1 - Regulator Output Voltage (1V/div.)

Trace 2 - Inductor Switching Node (5V/div.)

Trace 3 - Output Current (0.5 to 13 Amps) (20V/div.)

Trace 1 - Regulator Output Voltage (1V/div.)

Trace 3 - Regulator Output Current (20V/div.)

Trace1 - Regulator Output Voltage (10V/div.)

Trace 2 - Inductor Switching Node (5V/div.)