ADP1877ACPZ-R7 datasheet(18/32 Pages) AD

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ADP1877

Rev. 0 | Page 18 of 32

VIN

RILIM

DHx

DLx

SWx

ILIMx

RSENSE

ADP1877

VIN

RILIM

DHx

RAMP

DLx

SWx

ILIMx

RCSG

ADP1877

RRAMP

Figure 31. Accurate Current Limit Sensing

SETTING THE SLOPE COMPENSATION

Figure 32. Slope Compensation and CS Gain Connection

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

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

then 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 DL pin. This gain is sensed only during

power-up 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.

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).

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. Do not set VCSMAX

above 2.1 V to account for temperature and part-to-part

variations. Note that VCSMIN or VCSMAX is not the same as VCOMP,

which has a range of 0.75 V to 2.25 V. The following are

equations for VCSMIN and VCSMAX:

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 in between 10 μA and

200 μA. Make sure that the following condition is satisfied:

MIN

DSON

LPP

CSMIN

−

MAX

DSON

LPP

LOADMAX

CSMAX

)

(

RAMP

≤

−

≤

200

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.

The zero-current level voltage of the current sense amplifier is

0.75 V.

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

ILOADMAX is the maximum output DC load current.

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

exceed 1.1 MΩ. If the calculated RRAMP produces less than 10 μA,

then select a RRAMP value that produces between 10 μA and 20 μA.

Figure 32 illustrates the connection of the slope compensation

resistor RRAMP and the current sense gain resistor RCSG.

Table 6 shows the appropriate current sense gain settings for a

given RDSON maximum load current and a 33% inductor current

ripple. Because of the variation in RDSON of the power MOSFETs

(part-to-part variation and overtemperature) and the variation

of the inductors, the users must verify that VCOMP does not

exceed 2.2 V at the maximum output load current.

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