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Data Sheet

ADP1050

OPEN-LOOP INPUT VOLTAGE FEEDFORWARD

OPERATION

The ADP1050 can run in open-loop input voltage feedforward

operation mode. In this mode, the input voltage is sensed as the

feedforward signal for the generation of the PWM outputs.

As shown in Figure 22, the digital compensator output is modified

by a programmable modulation reference. The VF value, which

represents the input voltage, is fed into the feedforward ADC to

divide the modulation reference. The result of this division is then

fed into the PWM engine. The duty cycle value is in inverse

proportion to the input voltage.

DPWM

ENGINE

1/x

Σ-Δ

ADC

READ_VIN

REG 0x88

REG 0x35,

REG 0x36

FEED-

FORWARD

ADC

0.5V TO 1.6V

0V TO 1.6V

VIN_LOW

FLAG REG 0x7C[3]

REG 0xFE29[5]

VIN_UV_FAULT

FLAG REG 0x7C[4]

FROM THE VIN

SENSE CIRCUIT

MODULATION

REFERENCE

REG 0xFE63 AND REG 0xFE64

Figure 22. Open-Loop Feedforward Operation

Use the following equations to derive the output voltage equation:

D =

NOM

× (tREF × fSW)

and

VOUT =

IN ×

The output voltage can then be derived by

VOUT =

(

)

REF

NOM

where:

D is the duty cycle value.

VIN_NOM is the nominal input voltage.

VIN is the input voltage.

VOUT is the output voltage.

n is the turns ratio of the main transformer.

tREF is the modulation reference, which is set by Register 0xFE63

and Register 0xFE64.

fSW is the switching frequency.

In the equation to derive VOUT, the input voltage, VIN, is cancelled out.

Therefore, the output voltage does not change when the input

voltage changes.

based on the target output voltage and the nominal input voltage at

which the VF pin voltage is 1 V (see Figure 22).

The PWM settings for open-loop input voltage feedforward operation

are similar to those of general closed-loop operation. The falling edge

timings, rising edge timings, and modulation are set in the same

manner as for closed-loop operation, by using Register 0xFE3E to

modulation edges. Register 0xFE3D[6] enables open-loop feed-

forward operation. Register 0xFE3D[7] is used to enable the soft

start procedure for open-loop feedforward operation.

The flag settings for open-loop feedforward operation are also similar

to those of general closed-loop operation.

Because the output voltage is not regulated in the same manner as

in closed-loop operation, some settings, such as the VOUT setting,

the digital compensator settings, and the constant current mode

setting, are not functional. Other settings can be programmed in

a manner that is similar to general closed-loop operation.

OPEN-LOOP OPERATION

The ADP1050 can also run in open-loop operation mode. In this

mode, the rising edges and falling edges of the PWM outputs are

fixed during normal operation. Therefore, the output voltage varies

with the input voltage. The topologies include full bridge, half bridge,

and push pull converters.

The PWM settings for open-loop operation are different from those

of general closed-loop operation.

1. Set the rising edge timings and falling edge timings using

setting of ~50% is recommended for ease of zero voltage

switching operation. A phase shift function of 180° is

recommended to guarantee balanced PWM outputs.

2. Program Register 0xFE3C to a value of 0x00, which sets the

modulation limit to 0 µs.

3. For soft start, apply negative modulation to the falling edges

of the OUTA and OUTB outputs. The soft start of SR1 and

SR2 is not recommended.

4. Write 111111 to Register 0xFE67[5:4] and Register 0xFE67[1:0]

to set all PWM channels to follow open-loop operation. Set

soft start speed is specified by Register 0xFE09[4:3].

5. Always set Register 0xFE09[2] = 1. The soft start ramp time

is determined by tF2 − tR2.

Because the output voltage is not regulated, some of the settings,

such as the VOUT setting, digital compensator settings, and constant

current control, are not functional. Other settings can be pro-

grammed to be similar to those of general closed-loop operation.

Rev. A | Page 19 of 92

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