ADP1851ACPZ-R7 datasheet(19/24 Pages) AD

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Data Sheet

ADP1851

Rev. B | Page 19 of 24

Because CHF << CI, calculate CHF as follows:

(11)

Next, calculate the feedforward capacitor, CFF, assuming

RFF << RTOP.

(12)

Check that the calculated component values are reasonable.

For example, capacitors smaller than about 10 pF should be

avoided. In addition, RZ values less than 3 kΩ and CI values

greater than 10 nF should be avoided. If necessary, recalculate

the compensation network with a different starting value for

RTOP. If RZ is too small or CI is too big, start with a larger value

for RTOP. This compensation technique should yield a good

working solution.

When precise compensation is needed, use the ADIsimPower

design tool.

LOOP COMPENSATION—CURRENT MODE

Compensate the ADP1851 error voltage loop in current mode

using Type II compensation.

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 RAMP pin. To set the effective slope compensation,

connect a resistor (RRAMP) between the RAMP pin and the input

voltage (VIN). RRAMP is calculated by

RAMP

where:

L is the inductor value measured in µH.

RCS (mΩ) is the resistance of the current sense element between

SW and PGND (RDSON_MAX is the low-side MOSFET maximum

on resistance).

ACS is the current sense amplifier gain and is 3 V/V, 6 V/V,

or 12 V/V.

Thus, the voltage ramp amplitude, VRAMP, is:

RAMP

−

100

where 100 pF is the effective capacitance of the internal ramp

capacitor, CRAMP, with ±4% tolerance over the temperature and

VIN range.

The voltage at the RAMP pin is fixed at 0.2 V, and the current

going into RAMP should be between 10 µA and 160 µA. Make

sure that the following condition is satisfied:

μA

160

μA

≤

−

≤

RAMP

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

exceed 1.1 MΩ. If the calculated RRAMP value produces less than

10 µA, then select an RRAMP value that produces between 10 µA

and 15 µA.

Figure 27 illustrates the connection of the slope compensation

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

RCSG

RRAMP

VIN

RAMP

ADP1851

ILIM

RILIM

Figure 27. 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 then amplified

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

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 0.9 V to 2.2 V. Make

sure that the maximum VCOMP (VCOMPMAX) does not exceed 2.2 V

to account for temperature and part-to-part variations. See the

following equations for VCSMIN, VCSMAX, and VCOMPMAX.

MIN

DSON

CSMIN

−

MAX

DSON

LOADMAX

CSMAX

−

)

(

CSMAX

RAMP

COMPMAX

−

100

)

(

where:

VCSMIN is the minimum amplified voltage of the internal current

sense amplifier at zero output current.

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

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

zero current level voltage of the current sense amplifier is 0.75 V.

VCSMAX is the maximum amplified voltage of the internal current

sense amplifier at the maximum output current.

ILOADMAX is the maximum output dc load current.

RDSON_MAX is the low-side MOSFET maximum on resistance.

ACS is the current sense amplifier gain.

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