ADP5056ACCZ-R7 datasheet(21/31 Pages) AD

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Data Sheet

ADP5056

Rev. 0 | Page 21 of 31

APPLICATIONS INFORMATION

PROGRAMMING THE ADJUSTABLE OUTPUT

VOLTAGE

The output voltage of the ADP5056 is externally set by a resistive

voltage divider from the output voltage to the FBx pin. To limit

the degradation of the output voltage accuracy due to feedback

bias current, ensure that the bottom resistor in the divider is not

too large. A value of less than 50 kΩ is recommended.

The equation for the output voltage setting is

VOUT = VREF × (1 + (RTOP/RBOT))

where:

VOUT is the output voltage.

VREF is the feedback reference voltage, 0.6 V for Channel 1 to

Channel 3.

RTOP is the feedback resistor from VOUT to FBx.

RBOT is the feedback resistor from FBx to ground.

VOLTAGE CONVERSION LIMITATIONS

For a given input voltage, upper and lower limitations on the

output voltage exist due to the minimum on time and the

minimum off time.

The minimum on time limits the output voltage for a given

input voltage and switching frequency. The minimum on time

for Channel 1 to Channel 3 is 50 ns (maximum).

In FPWM mode, Channel 1 and Channel 2 can skip the switching

pulses to maintain the output regulation when the minimum on

time limit is exceeded. Careful selection of switching frequency is

required to avoid this condition.

To calculate the minimum output voltage in CCM for a given

input voltage and switching frequency, use the following

equation:

VOUT_MIN = VIN × tMIN_ON × fSW − (RDSON_HS − RDSON_LS) ×

IOUT_MIN × tMIN_ON × fSW − (RDSON_LS + RL) × IOUT_MIN

(1)

where:

VOUT_MIN is the minimum output voltage.

VIN is the input voltage.

tMIN_ON is the minimum on time.

fSW is the switching frequency.

RDSON_HS is the on resistance of the high-side MOSFET.

RDSON_LS is the on resistance of the low-side MOSFET.

IOUT_MIN is the minimum output current.

RL is the resistance of the output inductor.

The maximum output voltage for a given input voltage and

switching frequency is limited by the minimum off time and the

maximum duty cycle.

The maximum output voltage for a given input voltage and

switching frequency can be calculated using the following

equation:

VOUT_MAX = VIN × (1 − tMIN_OFF × fSW) − (RDSON_HS − RDSON_LS) ×

IOUT_MAX × (1 − tMIN_OFF × fSW) − (RDSON_LS + RL) × IOUT_MAX (2)

where:

tMIN_OFF is the minimum off time.

IOUT_MAX is the maximum output current.

As shown in Equation 1 and Equation 2, reducing the switching

frequency eases the minimum on time and off time limitations.

CURRENT-LIMIT SETTING

The ADP5056 has two selectable current-limit thresholds for

Channel 1, Channel 2, and Channel 3. Ensure that the selected

current-limit value is larger than the peak current of the inductor

(IPEAK) for the current-limit configuration for all channels.

SOFT START SETTING

The buck regulators in the ADP5056 include soft start circuitry

that ramps the output voltage in a controlled manner during

startup, thereby limiting the inrush current. To set the soft start

time to a value of 2.2 ms or 17.3 ms, connect a resistor from the

CFG2 pin to ground (see the Soft Start section).

INDUCTOR SELECTION

The inductor value is determined by the switching frequency,

input voltage, output voltage, and inductor ripple current. Using

a small inductor value yields faster transient response but may

degrade efficiency due to the larger inductor ripple current.

Using a large inductor value yields a smaller ripple current and

improved efficiency but results in slower transient response.

Thus, a trade-off must be made between transient response and

efficiency. As a guideline, the inductor peak-to-peak ripple

current, ΔIL, is typically set to a value from 30% to 40% of the

maximum load current. Use the following equation to calculate

the inductor value:

L = ((VIN − VOUT) × D)/(ΔIL × fSW)

where:

VOUT is the output voltage.

D is the duty cycle (D = VOUT/VIN).

ΔIL is the inductor ripple current.

The ADP5056 has internal slope compensation in the current

loop to prevent subharmonic oscillations when the duty cycle is

greater than 50%.

Use the following equation to calculate the peak inductor

current:

IPEAK = IOUT + (ΔIL/2)

The saturation current of the inductor must be larger than the

peak inductor current. For ferrite core inductors with a fast

saturation characteristic, ensure that the saturation current rating

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