ADP1853ACPZ-R7 datasheet(21/28 Pages) AD

21 / 28 page

Data Sheet

ADP1853

Rev. 0 | Page 21 of 28

Because CHF << CI, calculate CHF as follows:

(9)

Next, calculate the feedforward capacitor, CFF, assuming RFF <<

RTOP:

(10)

Check that the calculated component values are reasonable.

For instance, 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 ADP1853 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 resistance of the current sense element between

CS and PGND (for instance, 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 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 an RRAMP value that produces between

10 µA and 15 µA.

Figure 28 illustrates the connection of the slope compensation

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

Figure 28. Slope Compensation and CS Gain Connection

ADP1853

ILIM

RILIM

RCSG

RAMP

RRAMP

VIN

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