ADP1876-EVALZ AD [Analog Devices], ADP1876-EVALZ Datasheet - Page 22

• Manufacturer Part Number: ADP1876-EVALZ
• Description: 600 kHz Dual Output Synchronous Buck
• Manufacturer: AD [Analog Devices]

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, Z
Because C
At the crossover frequency, the open-loop transfer function is
unity of 0 dB, H (f
tion 3, Z
The zero produced by R
At the crossover frequency, Equation 4 can be shown as
Combining Equation 5 and Equation 7 and solving for R
gives
Choose the crossover and zero frequencies as follows:
Substituting Equation 2, Equation 9, and Equation 10 into
Equation 8 yields
where:
g
A
R
V
And combining Equation 6 and Equation 10 yields
ADP1876
m
DSON
CS
REF
is the transconductance of the error amplifer, 500 µS.
is the current sense gain of 3 V/V, 6 V/V, 12 V/V, or 24 V/V.
= 0.6 V.
C
Z
Z
Z
Z
R
R
f
f
f
is the on resistance of the low-side MOSFET.
CROSS
ZERO
ZERO
COMP
COMP
FILTER
COMP
COMP
COMP
COMP
COMP
C2
=
=
=
(
(
=
=
is small relative to C
=
=
f
=
f
12
f
at the crossover frequency can be written as
SW
f
CROSS
. 0
2
CROSS
CROSS
f
R
π
sC
π
CROSS
97
4
COMP
R
R
1
OUT
COMP
f
×
COMP
CROSS
)
CROSS
)
=
=
A
+
=
+
CS
R
f
2
(
f
1
48
) = 1. Combining Equation 1 and Equa-
×
SW
COMP
ZERO
2
sC
×
×
COMP
g
π
f
C
R
CROSS
m
COMP
×
1
COMP
DSON
×
×
×
f
and C
(
CROSS
G
2
COMP




CS
g
π
=
f
m
2
CROSS
×
1
π
×
)(
COMP
f
, Z
+
×
CROSS
G
g
C
sR
f
f
CS
COMP
2
m
CROSS
CROSS
OUT
+
FILTER
COMP
is
sC
V
)
f
×
can be simplified to
REF
ZERO
COMP
×
(




C
V
×
is given by
×
OUT
OUT




C
2
C
V
COMP
OUT
×
REF
)
V
V
OUT
REF
×
V
OUT
)
COMP




(10)
(12)
Rev. A | Page 22 of 24
(11)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
Note that the previous simplified compensation equations for
R
margin assuming that the compensation ramp current is ideal.
Varying the ramp current, or deviating the ramp current from
ideal, can affect f
Lastly, set C
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,
C
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, R
typically 1.2 nF to 3 nF.
R
And C
where:
L
side MOSFETs, typically 3 nH, and is package dependent.
C
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 R
In most applications, a component size 0805 for R
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.
MOSFET
COMP
SNUB
OSS
GD
, of the external MOSFET as well as the parasitic inductance
SNUB
is the total output capacitance of the high-side and low-side
R
C
R
20
can be estimated by
1
and C
SNUB
SNUB
SNUB
SNUB
is the total parasitic inductance of the high-side and low-
×
is
C
= V
can be estimated by
≅
COMP
≅
C2
COMP
C
2
to
IN
OSS
2
CROSS
L
× C
yield reasonable results in f
≤
MOSFET
C
C
OSS
SNUB
C
and phase margin.
2
SNUB
≤
× f
10
1
is typically 2 Ω to 4 Ω, and C
SW
×
C
COMP
CROSS
Data Sheet
SNUB
and phase
is sufficient.
SNUB
(13)
is