ADP1851ACPZ-R7 datasheet(20/24 Pages) AD

20 / 24 page

ADP1851

Data Sheet

Rev. B | Page 20 of 24

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

(dB)

PHASE

–180°

–270°

CHF

RTOP

RBOT

VOUT

INTERNAL

VREF

COMP

–1

SLO

–1

SLO

Figure 28. Type II Compensation

For Type II compensation, use the circuit shown in Figure 28.

Calculate the compensation resistor, RZ, with the following

equation:

OUT

TOP







(13)

where:

fCO is 1/10 of fSW.

RS = ACS × RDSON_MIN.

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

12 V/V, set by the gain resistor between DL and PGND.

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

becomes





Next, choose the compensation capacitor to set the compensa-

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

1/2 of the LC resonant frequency.





(14)





(15)

Solving for CI in Equation 14 yields





(16)

Solving for CI in Equation 15 yields





(17)

Use the larger value of CI from Equation 16 or Equation 17.

Because of the finite output current drive of the error amplifier,

CI must be less than 10 nF. If it is larger than 10 nF, choose a

larger RTOP and recalculate RZ and CI until CI is less than 10 nF.

Next, choose the high frequency pole, fP1, to be 1/2 of fSW.



(18)

Because CHF << CI,





(19)

Combine Equation 18 and Equation 19, and solve for CHF.





(20)

For maximally precise compensation solutions, use the

ADIsimPower design tool.

SWITCHING NOISE AND OVERSHOOT REDUCTION

To reduce voltage ringing and noise, it is recommended that an

RC snubber be added between SW and PGND for high current

applications, as illustrated in Figure 29.

In most applications, RSNUB is typically 2 Ω to 4 Ω, and CSNUB is

typically 1.2 nF to 3 nF.

The size of the RC snubber components must be chosen

correctly to handle the power dissipation. The power dissipated

in RSNUB is

SNUB





In most applications, a component size of 0805 for RSNUB is

sufficient. The RC snubber does not reduce the voltage over-

shoot. A resistor, RRISE in Figure 29, at the BST pin helps to reduce

overshoot and is generally between 2 Ω and 4 Ω. Adding a resistor

in series, typically between 2 Ω and 4 Ω, with the gate driver also

helps to reduce overshoot. If a gate resistor is added, RRISE is

not needed.

VIN

ADP1851

RRISE

VOUT

CSNUB

COUT

RSNUB

BST

PGND

Figure 29. Application Circuit with a Snubber

ADP1851ACPZ-R7 数据表(PDF) 20 Page - Analog Devices

ADP1851ACPZ-R7 数据表(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

数据表下载