ADP1876ACPZ-R7 datasheet(18/24 Pages) AD

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ADP1876

Data Sheet

Rev. A | Page 18 of 24

Figure 28. Accurate Current-Limit Sensing

SETTING THE SLOPE COMPENSATION

In a current mode control topology, slope compensation is

needed to prevent subharmonic oscillations in the inductor

current and to maintain a stable output. The external slope

compensation is implemented by summing the amplified sense

signal and a scaled voltage at the RAMPx pin. To implement the

slope compensation, connect a resistor between RAMPx and

the input voltage. The resistor, RRAMP, is calculated by

MAX

DSON

RAMP

where:

3.6 × 1010 is an internal parameter.

L is the inductance of the inductor.

RDSON_MAX is the the low-side MOSFET maximum on resistance.

ACS is the gain, either 3 V/V, 6 V/V, 12 V/V, or 24 V/V, of the

current sense amplifier (see the Setting the Current Sense Gain

section for more details).

Keep in mind that RDSON is temperature dependent and can vary

as much as 0.4%/oC. Choose RDSON at the maximum operating

temperature. The voltage at RAMPx is fixed at 0.2 V, and the

current going into RAMPx should be between 6 µA and 200 µA.

Ensure that the following condition is satisfied:

μA

200

μA

≤

−

≤

RAMP

For instance, with an input voltage of 12 V, RRAMP should not

exceed 1.9 MΩ. If the calculated RRAMP produces less than 6 µA,

select an RRAMP value that produces between 6 µA and 20 µA.

Figure 29 illustrates the connection of the slope compensation

resistor, RRAMP, and the current sense gain resistor, RCSG.

Figure 29. Slope Compensation and CS Gain Connection

SETTING THE CURRENT SENSE GAIN

The voltage drop across the external low-side MOSFET is

sensed by a current sense amplifier by multiplying the peak

inductor current and the RDSON of the MOSFET. The result is

amplified by a gain factor of either 3 V/V, 6 V/V, 12 V/V, or

24 V/V, which is programmable by an external resistor, RCSG,

connected to the DLx pin. This gain is sensed during power-up

only and not during normal operation. The amplified voltage is

summed with the slope compensation ramp voltage and fed

into the PWM controller for a stable regulation voltage.

The voltage range of the internal node, VCS, is between 0.4 V

and 2.2 V. Select the current sense gain such that the internal

minimum amplified voltage (VCSMIN) is above 0.4 V and the

maximum amplified voltage (VCSMAX) is 2.1 V. Note that VCSMIN

or VCSMAX is not the same as VCOMP, which has a range of 0.85 V

to 2.3 V. Make sure that the maximum VCOMP (VCOMPMAX) does

not exceed 2.2 V to account for temperature and device to

device variations. The following are equations for VCSMIN,

VCSMAX, and VCOMPMAX:

MIN

DSON

LPP

CSMIN

−

MAX

DSON

LPP

LOADMAX

CSMAX

)

(

)

CSMAX

RAMP

COMPMAX

−

where:

VCSMIN is the minimum amplified voltage of the internal current

sense amplifier at zero output current.

VCSMAX is the maximum amplified voltage of the internal current

sense amplifier at maximum output current.

RDSON_MIN is the the low-side MOSFET minimum on resistance.

ILPP is the peak-to-peak ripple current in the inductor.

ILOADMAX is the maximum output dc load current.

VCOMPMAX is the maximum voltage at the COMP pin.

ACS is the current sense gain of either 3 V/V, 6 V/V, 12 V/V, or

24 V/V set by the gain resistor between DLx and PGNDx.

25 pF is an internal parameter.

tON is on time for the high-side driver (DH).

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 a sufficient

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

equivalent series resistance (ESR) that is low enough to mitigate

input voltage ripple. For the usual current ranges for these con-

verters, 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).

VIN

ADP1876

DHx

SWx

ILIMx

DLx

RILIM

RSENSE

PGNDx

VIN

ADP1876

DHx

SWx

ILIMx

DLx

RILIM

RCSG

RAMP

RRAMP

PGNDx

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