ADP1876ACPZ-R7 datasheet(22/24 Pages) AD

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ADP1876

Data Sheet

Rev. A | Page 22 of 24

Because the zero produced by the ESR of the output capacitor is

not needed to stabilize the control loop, assuming ESR is small,

the ESR is ignored for analysis. Then, ZFILTER is given by

OUT

FILTER

(3)

Because CC2 is small relative to CCOMP, ZCOMP can be simplified to

COMP

(4)

At the crossover frequency, the open-loop transfer function is

unity of 0 dB, H (fCROSS) = 1. Combining Equation 1 and Equa-

tion 3, ZCOMP at the crossover frequency can be written as

)

)(

(

)

(

REF

OUT

CROSS

COMP

(5)

The zero produced by RCOMP and CCOMP is

COMP

ZERO

(6)

At the crossover frequency, Equation 4 can be shown as

CROSS

ZERO

CROSS

COMP

CROSS

COMP

)

(

(7)

Combining Equation 5 and Equation 7 and solving for RCOMP

gives

)

(

)

(

REF

OUT

CROSS

ZERO

CROSS

COMP

(8)

Choose the crossover and zero frequencies as follows:

CROSS

(9)

CROSS

ZERO

(10)

Substituting Equation 2, Equation 9, and Equation 10 into

Equation 8 yields

























REF

OUT

CROSS

DSON

COMP

(11)

where:

gm is the transconductance of the error amplifer, 500 µS.

ACS is the current sense gain of 3 V/V, 6 V/V, 12 V/V, or 24 V/V.

RDSON is the on resistance of the low-side MOSFET.

VREF = 0.6 V.

And combining Equation 6 and Equation 10 yields

CROSS

COMP

(12)

Note that the previous simplified compensation equations for

RCOMP and CCOMP yield reasonable results in fCROSS and phase

margin assuming that the compensation ramp current is ideal.

Varying the ramp current, or deviating the ramp current from

ideal, can affect fCROSS and phase margin.

Lastly, set CC2 to

COMP

≤

(13)

SWITCHING NOISE AND OVERSHOOT REDUCTION

In any high speed step-down regulator, high frequency noise

(generally in the range of 50 MHz to 100 MHz) and voltage

overshoot are always present at the gate, the switch node (SW),

and the drains of the external MOSFETs. The high frequency

noise and overshoot are caused by the parasitic capacitance,

CGD, of the external MOSFET as well as the parasitic inductance

of the gate trace and the packages of the MOSFETs. When the

high current is switched, electromagnetic interference (EMI) is

generated, which can affect the operation of the surrounding

circuits. To reduce voltage ringing and noise, it is recommended

to add an RC snubber between SWx and PGNDx for high current

applications, as illustrated in Figure 32.

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

typically 1.2 nF to 3 nF.

RSNUB can be estimated by

OSS

MOSFET

SNUB

≅

And CSNUB can be estimated by

OSS

SNUB

≅

where:

LMOSFET is the total parasitic inductance of the high-side and low-

side MOSFETs, typically 3 nH, and is package dependent.

COSS is the total output capacitance of the high-side and low-side

MOSFETs given in the MOSFET data sheet.

The size of the RC snubber components need to be chosen

correctly to handle the power dissipation. The power dissipated

in RSNUB is

RSNUB = VIN2 × CSNUB × fSW

In most applications, a component size 0805 for RSNUB is sufficient.

However, the use of an RC snubber reduces the overall efficiency,

generally by an amount in the range of 0.1% to 0.5%. The RC

snubber does not reduce the voltage overshoot.

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