ADP1877ACPZ-R7 datasheet(19/32 Pages) AD

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ADP1877

Rev. 0 | Page 19 of 32

Table 6. CS Gain Setting Selection Table for Some Popular Configurations

ILPP = 33% Load

ACS = 3

ACS = 6

ACS = 12

ACS = 24

RDSON (mΩ)

Load (A)

VCS Min (V)

VCS Max (V)

VCS Min (V)

VCS Max (V)

VCS Min (V)

VCS Max (V)

VCS Min (V)

VCS Max (V)

1.5

0.73

0.9

0.71

1.01

0.7

1.3

0.6

1.80

0.73

0.9

0.70

1.10

0.7

1.4

0.73

0.9

0.71

1.03

0.7

1.3

0.72

1.0

0.69

1.17

0.6

1.6

0.71

1.0

0.68

1.27

0.6

1.8

0.72

1.0

0.68

1.24

0.6

1.7

0.70

1.1

0.65

1.45

0.69

1.2

0.63

1.59

0.68

1.3

0.61

1.76

0.68

1.2

0.61

1.73

0.69

1.2

0.63

1.62

0.68

1.3

0.60

1.80

0.65

1.4

0.66

1.4

0.67

1.3

100

0.65

1.4

120

0.63

1.6

INPUT CAPACITOR SELECTION

The input current to a buck converter is a pulse waveform. It is

zero when the high-side switch is off and approximately equal

to the load current when it is on. The input capacitor carries the

input ripple current, allowing the input power source to supply

only the direct current. The input capacitor needs sufficient

ripple current rating to handle the input ripple, as well as an

ESR that is low enough to mitigate input voltage ripple. For the

usual current ranges for these converters, it is good practice to

use two parallel capacitors placed close to the drains of the

high-side switch MOSFETs (one bulk capacitor of sufficiently

high current rating and a 10 μF ceramic decoupling capacitor,

typically).

Select an input bulk capacitor based on its ripple current rating.

First, determine the duty cycle of the output.

OUT

D =

The input capacitor RMS ripple current is given by

)

(

RMS

−

where:

IO is the output current.

D is the duty cycle

The minimum input capacitance required for a particular load is

ESR

MIN

)

(

)

(

−

where:

VPP is the desired input ripple voltage.

RESR is the equivalent series resistance of the capacitor.

If an MLCC capacitor is used, the ESR is near 0, then the

equation is simplified to

MIN

−

)

(

The capacitance of MLCC is voltage dependent. The actual

capacitance of the selected capacitor must be derated accordingly.

In addition, add more bulk capacitance, such as by using

electrolytic or polymer capacitors, as necessary for large step

load transisents. Make sure the current ripple rating of the bulk

capacitor exceeds the minimum input current ripple of a

particular design.

INPUT FILTER

Normally the input pin, VIN, with a 0.1 μF or greater value

bypass capacitor to AGND, is sufficient for filtering out any

unwanted switching noise. However, depending on the PCB

layout, some switching noises can be passed down to the ADP1877

internal circuitry; therefore, it is recommended to have a low

pass filter at the VIN pin. Connecting a resistor, between 2 Ω

and 5 Ω, in series with VIN and a 1 μF ceramic capacitor

between VIN and AGND creates a low pass filter that effectively

filters out any unwanted glitches caused by the switching

regulator. Keep in mind that the input current could be larger than

100 mA when driving large MOSFETs. A 100 mA across a 5 Ω

resistor creates a 0.5 V drop, which is the same voltage drop in

VCCO. In this case, a lower resistor value is desirable.

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