ADP1853ACPZ-R7 datasheet(22/28 Pages) AD

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ADP1853

Data Sheet

Rev. 0 | Page 22 of 28

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 0.85 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.

VCOMPMAX is the maximum voltage at the COMP pin.

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

Replace RDSON with the resistance value of the current sense

element, RCS, if it is used.

Type II Compensation

Figure 29. Type II Compensation

In this case, use the circuit shown in Figure 29. Calculate the

compensation resistor, RZ, with the following equation:

OUT

TOP







(11)

where:

fCO is chosen to be 1/10 of fSW.

RS = ACS × RDSON_MIN.

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

set by the gain resistor between DL and PGND.

RDSON_MIN is the low-side MOSFET minimum on resistance.

If the current is sensed on a current sense resistor, RCS, then RCS

becomes





Next, choose the compensation capacitor to set the compensa-

tion zero, fZ1, to the lesser of 1/5 of the crossover frequency or ½

of the LC resonant frequency





(12)





(13)

Solving for CI in Equation 12 yields





(14)

Solving for CI in Equation 13 yields





(15)

(dB)

PHASE

–180°

–270°

CHF

RTOP

RBOT

VOUT

INTERNAL

VREF

COMP

–1

SLO

–1

SLO

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