ADP2140 datasheet(24/32 Pages) AD

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ADP2140

Data Sheet

Rev. B | Page 24 of 32

CH1 500mV

CH2 500mV

M40.0µs

A CH3

1.16V

T 10.00%

CH3 2.00V

LDO OUTPUT

BUCK OUTPUT

EN1

Figure 84. Individual Activation Mode, EN1 and EN2 Pins Tied Together

POWER-GOOD FUNCTION

The ADP2140 power-good (PG) pin indicates the state of the

monitored output voltages. The PG function is the logical AND

of the state of both outputs. The PG function is an active high,

open-drain output, requiring an external pull-up resistor typically

supplied from the I/O supply rail, as shown in . When the sensed

output voltages are below 92% of their nominal value, the PG pin is

held low. When the sensed output voltages rise above 92% of

the nominal levels, the PG line is pulled high after tRESET. The

PG pin remains high as long as the sensed output voltages are

above 86% of the nominal output voltage levels.

The typical PG delay when the buck is in PWM mode is 5 ms.

When the part is in PSM mode, the PG delay is load dependent

because the internal clock is disabled to reduce quiescent current

during the sleep stage. PG delay varies from hundreds of micro-

seconds at 10 mA, up to seconds at current loads of less than 10 μA.

CH1 2.00V

CH2 2.00V

CH4 2.00V

M2.00ms

A CH1

2.20V

T 10.20%

CH3 2.00V

EN1

BUCK

LDO

Figure 85. Typical PG Timing

EXTERNAL COMPONENT SELECTION

The external component selection for the ADP2140 application

circuit that is shown in Table 8, Table 9, and Figure 86 is

dependent on input voltage, output voltage, and load current

requirements. Additionally, trade-offs between performance

parameters such as efficiency and transient response can be

made by varying the choice of external components.

SELECTING THE INDUCTOR

The high frequency switching of the ADP2140 allows the selection

of small chip inductors. The inductor value affects the transition

between CFM to PSM, efficiency, output ripple, and current limit

values. Use the following equation to calculate the inductor ripple

current:

OUT

−

)

(

where:

fSW is the switching frequency (3 MHz typical).

L is the inductor value.

The dc resistance (DCR) value of the selected inductor affects

efficiency, but a decrease in this value typically means an increase

in root mean square (rms) losses in the core and skin. As a

minimum requirement, the dc current rating of the inductor

should be equal to the maximum load current plus half of the

inductor current ripple, as shown by the following equation:

)

(

)

(

MAX

LOAD

OUTPUT CAPACITOR

Output capacitance is required to minimize the voltage over-

shoot and ripple present on the output. Capacitors with low

equivalent series resistance (ESR) values are recommended to

produce low output ripple. For good stability over temperature,

use capacitors such as the X5R or X7R dielectric. Do not use the

Y5V and Z5U capacitors; they are not suitable for this application

because of their large variation in capacitance over temperature

and dc bias voltage. The minimum output capacitance (COUT_MIN)

is determined by the following VRIPPLE and COUT_MIN equations.

For acceptable maximum output voltage ripple,

VRIPPLE = ΔIL × (ESRCOUT + 1/(8 × fSW × COUT_MIN))

Therefore,

COUT_MIN = ΔIL/(8 × fSW × (VRIPPLE − ΔIL × ESRCOUT))

where:

VRIPPLE is allowable peak-to-peak output voltage ripple in V.

ΔIL is the inductor ripple current in A.

ESRCOUT is the equivalent series resistance of the capacitor in Ω.

fSW is the converter switching frequency in Hz.

Increasing the output capacitor has no effect on stability and

increasing the output capacitance may further reduce output

ripple and enhance load transient response. When choosing this

value, it is also important to account for the loss of capacitance

due to output voltage dc bias. For recommended 10 μF

capacitors, please refer to Table 9.

INPUT CAPACITOR

Input capacitance is required to reduce input voltage ripple; there-

fore, place the input capacitor as close as possible to the VINx pins.

As with the output capacitor, a low ESR X7R- or X5R-type

capacitor is recommended to help minimize the input voltage

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