Figure 6: Normal Operation showing Output Inductor Ripple Current

Figure 7: Normal Operation showing Output Inductor Ripple Current

10111).

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 cur-

rent 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 capacitors voltage

to reduce total output voltage excursions during changes

in load.

Holding tolerance to 1% allows the error amplifiers refer-

ence voltage to be targeted +25mV high without compro-

mising DC accuracy. A “Droop Resistor”, implemented

through a PC board trace, connects the Error Amps feed-

ries the output current. With no load, there is no DC drop

across this resistor, producing an output voltage tracking

the Error amps, including the +25mV offset. When the full

load current is delivered, a 50mV drop is developed across

this resistor. This results in output voltage being offset -

25mV low.

The result of Adaptive Voltage Positioning is that addition-

al margin is provided for a load transient before reaching

the output voltage specification limits. When load current

suddenly increases from its minimum level, the output

capacitor is pre-positioned +25mV. Conversely, when load

current suddenly decreases from its maximum level, the

output capacitor is pre-positioned -25mV (see Figures 8, 9,

and 10). 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 8: Output Voltage Transient Response to a 14A load pulse,

2.825V (DAC = 10111).

Trace 1 - GATE(H) (10V/div)

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

Trace 3 -Load Current (5A/div)

Trace 3 -Load Current (5A/10mV/div)

Trace 4 - VOUT (100mV/div)

Trace 1 - GATE(H) (10/div)

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

Trace 3 - Output Inductor Ripple Current (2A/div)

Trace 1 GATE (H) (10V/div)

Trace 2 Inductor Switching Node (5V/div)

Trace 3 Output Inductor Ripple Current (2A/div)

Trace 4 VOUT ripple (20mV/div)

9

Application Information: continued