ADP5140WACCZ-R7 datasheet(57/128 Pages) AD

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Data Sheet

ADP5140

Rev. 0 | Page 57 of 128

Output Capacitor Selection for the LDO Regulators

The output capacitance of the LDO regulators depends mainly

on the load current. Generally, a large load current requires a

large output capacitance to achieve a stable output voltage. A

higher capacitor value improves the transient response of the

LDO regulators to large changes in the load current.

Ceramic capacitors with a typical value of 1 μF to 10 μF are

recommended as the output capacitors of the LDO regulators

in the ADP5140.

LOW OUTPUT NOISE DESIGN OF BUCK4

In typical system application of the ADP5140, BUCK4 provides

power for the power amplifier, which requires low noise input.

BUCK4 in the ADP5140 optimizes the internal analog blocks and

uses low noise reference architecture to achieve lower output

noise. When the system design requires BUCK4 of the ADP5140

to power the power amplifier directly without the LDO regulator,

it is highly recommended to add an additional secondary LC

filter after the primary LC filter to filter the fundamental

switching ripple further and achieve lower output noise in the

noise sensitive frequency range of the power amplifier.

Because the secondary LC filter generates voltage drop when

the load increases, an inductor with a small dc current

resistance (DCR) is recommended to minimize the voltage

drop, especially for a high current application.

There is a hybrid feedback method, as shown in Figure 95,

which provides an adequate stability margin and maintains the

output accuracy over all load conditions in the application

where the secondary LC filter of BUCK4 is added.

CSEC_F

RESR_F

LSEC_F

VOUT4

SW4

FB4

PGND

Figure 95. Hybrid Feedback Method of BUCK4 With Secondary LC Filter

To maintain the loop stability of BUCK4 when using the hybrid

feedback method, the value constrains of the components shown

in the following equation must be met:

SEC F

ESR F

×>

+×

where:

RF is the feedback resistor.

CF is the feedback capacitor.

LSEC_F is the inductor of the secondary LC filter.

CSEC_F is the capacitor of the secondary LC filter.

RESR_F is the equivalent series resistance of CSEC_F.

RL is the load resistance.

Note that larger RF and CF values degrade the load transient

performance of BUCK4 because RF and CF work as an RC filter

of the output voltage during load transient. It is recommended

that RF × CF be 20% to 30% larger than the minimum limitation

value shown in the previous equation to balance the loop

stability and load transient performance.

VOLTAGE CONVERSION LIMITATIONS

There is a minimum on time and a minimum off time for each

switching regulator. The voltage conversion between the input

voltage and output voltage of each switching regulator has

limitations.

Buck Regulator

The minimum output voltage of a buck regulator for a given

input voltage and switching frequency is constrained by the

minimum on time of the buck regulator and can be calculated

using the following equation:

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

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

where:

VOUT_MIN is the minimum output voltage.

VIN is the input voltage.

tMIN_ON is the minimum on time.

RDSON_HS is the high-side MOSFET on resistance.

RDSON_LS is the low-side MOSFET on resistance.

IOUT_MIN is the minimum output current.

RL is the series resistance of output inductor.

The maximum output voltage of a buck regulator for a given

input voltage and switching frequency is constrained by the

minimum off time of the buck regulator and 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

where:

VOUT_MAX is the maximum output voltage.

tMIN_OFF is the minimum off time.

IOUT_MAX is the maximum output current.

Boost Regulator

The maximum input voltage of the boost regulator for a given

output voltage and switching frequency is constrained by the

minimum on time of the boost regulator and can be calculated

using the following equation:

VIN_MAX = VOUT × (1 − tMIN_ON × fSW) + IOUT_MIN × (RDSON_MAIN ×

(1 − tMIN_ON × fSW) + RL + tMIN_ON × fSW × RDSON_SYNC)/

(1 − tMIN_ON × fSW)

where:

VIN_MAX is the maximum input voltage.

VOUT is the output voltage.

tMIN_ON is the minimum on time.

IOUT_MIN is the minimum output current.

RDSON_MAIN is the main MOSFET on resistance.

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