ADP1613ARMZ-R7 datasheet(15/28 Pages) AD

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ADP1612/ADP1613

Rev. A | Page 15 of 28

The capacitor, C2, is chosen to cancel the zero introduced by

output capacitance, ESR.

To determine the crossover frequency, it is important to note

that, at that frequency, the compensation impedance (ZCOMP)

is dominated by a resistor, and the output impedance (ZOUT) is

dominated by the impedance of an output capacitor. Therefore,

when solving for the crossover frequency, the equation (by

definition of the crossover frequency) is simplified to

Solve for C2 as follows:

COMP

OUT

ESR

(19)

OUT

COMP

MEA

OUT

(15)

For low ESR output capacitance such as with a ceramic

capacitor, C2 is optional. For optimal transient performance,

RCOMP and CCOMP might need to be adjusted by observing the

load transient response of the ADP1612/ADP1613. For most

applications, the compensation resistor should be within the

range of 4.7 kΩ to 100 kΩ and the compensation capacitor

should be within the range of 100 pF to 3.3 nF.

where:

fC is the crossover frequency.

RCOMP is the compensation resistor.

SOFT START CAPACITOR

Solve for RCOMP,

MEA

OUT

COMP

)

(

(16)

Upon startup (EN ≥ 1.6 V), the voltage at SS ramps up slowly

by charging the soft start capacitor (CSS) with an internal 5 μA

current source (ISS). As the soft start capacitor charges, it limits

the peak current allowed by the part to prevent excessive over-

shoot at startup. The necessary soft start capacitor, CSS, for a

specific overshoot and start-up time can be calculated for the

maximum load condition when the part is at current limit by:

where:

VFB = 1.235 V.

GMEA = 80 μA/V.

GCS = 13.4 A/V.

OUT

COMP

)

(

4746

(17)

(20)

where:

ISS

= 5 μA (typical).

VSS

= 1.2 V.

Δt = startup time, at current limit.

Once the compensation resistor is known, set the zero formed

by the compensation capacitor and resistor to one-fourth of the

crossover frequency, or

COMP

(18)

If the applied load does not place the part at current limit, the

necessary CSS will be smaller. A 33 nF soft start capacitor results

in negligible input current overshoot at start up, and therefore is

suitable for most applications. However, if an unusually large

output capacitor is used, a longer soft start period is required

to prevent input inrush current.

where CCOMP is the compensation capacitor.

RCOMP

CCOMP

COMP

ERROR

AMPLIFIER

VBG

Conversely, if fast startup is a requirement, the soft start

capacitor can be reduced or removed, allowing the

ADP1612/ADP1613 to start quickly, but allowing greater

peak switch current.

Figure 35. Compensation Components

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