ADP1048WARQZ-R7 datasheet(18/82 Pages) AD

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ADP1048W

Data Sheet

Fast Loop Mode

During transients, a fast loop mode is enabled to allow for faster

loop responses. Typical timing can be seen in Figure 12. The

fast loop mode has separate settings and can be programmed to

respond quickly to load transients. The user can disable the fast

loop mode if it is not required by the application.

Figure 12. Fast Loop for Transient Response Improvement

When fast loop mode is enabled and the feedback output voltage

is out of range from the desired reference value (programmable

band of 1.5%, 3%, 6%, or 12%, set in Register 0xFE24), the

ADP1048W enters fast loop mode.

To ensure a smooth transition, the ADP1048W switches from

the regular filter to the fast loop filter at the zero crossing of the

rectified input voltage. When the output voltage returns to

regulation within the programmed band, the controller switches

back (after a programmable delay of 0 to 7 half line cycles) to

the normal loop at the next zero crossing of the rectified input

voltage.

If the output voltage does not return to regulation within the

programmed band after a fixed time of 630 ms, the control loop

automatically switches back to the normal loop.

In the normal compensation loop, the sampling frequency of

the output voltage is the same as the ripple oscillation frequency

(which is commonly 100 Hz or 120 Hz).

During fast loop operation, the feedback voltage is sampled

at 1.5 kHz, and the fast filter is applied to regulate the output

voltage. The output voltage is averaged and decimated at

1.5 kHz (see Figure 13).

Figure 13. Fast Loop Operation

Based on the requirements of the application, the user can enable

or disable the fast loop mode by programming Register 0xFE24.

It is recommended that fast loop mode be enabled for the

ADP1048W during large load transients. The fast loop mode

settings are also used during soft start, even when the fast loop

is disabled.

PULSE-WIDTH MODULATION

The ADP1048W can implement either leading edge or trailing

edge modulation. Trailing edge modulation is the more popular

modulation scheme. Using trailing edge modulation, the rms

ripple current in the bulk capacitors can be reduced when used

with downstream converter synchronization. It is recommended

that the Analog Devices, Inc., GUI software be used to program

PWM (see the Software GUI section).

DUTY CYCLE MINIMUM/MAXIMUM LIMITS

The ADP1048W allows the user to program the minimum off

time and the minimum on time for the PWM outputs separately,

thereby allowing the minimum and maximum duty cycles to be set.

The minimum off time represents the minimum time that the

PWM is low during each switching cycle. It can be programmed

from 40 ns to 1.2 μs in steps of 80 ns using Register 0xFE15,

Bits[3:0]. In this way, the maximum duty cycle can be clamped

between 96% and 99.8% at the minimum frequency and

between 48.8% and 96.8% at the maximum frequency.

The minimum on time is the smallest PWM pulse that the mod-

ulator generates on the PWM output. It can be programmed

from 0 ns to 1200 ns in steps of 80 ns using Register 0xFE15,

Bits[7:4].

PROGRAMMABLE

RANGE

ILOAD

VOUT

FAST LOOP

VREC

PROGRAM-

MABLE

DELAY

(0 TO 7

HALF LINE

CYCLES)

VFB

VFB ERROR

FAST LOOP

VFB FAST ERROR

NORMAL FILTER

@ 100Hz

FAST FILTER

@ 1.5kHz

Rev. 0 | Page 18 of 82

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