Datasheet
ADP1851 Data Sheet
Rev. 0 | Page 20 of 24
A
CS
is the current sense amplifier gain.
V
COMPMAX
is the maximum voltage at the COMP pin.
t
ON
is the high-side driver (DH) on time.
Replace R
DSON
with the resistance value of the current sense
element, R
CS
, if it is used.
Type II Compensation
Figure 28. Type II Compensation
For Type II compensation, use the circuit shown in Figure 28.
Calculate the compensation resistor, R
Z
, with the following
equation:
CO
OUT
S
TOPZ
fCRRR
2
(13)
where:
f
CO
is 1/10 of f
SW
.
R
S
= A
CS
× R
DSON_MIN
.
A
CS
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, R
CS
, then R
S
becomes
CSCSS
RAR
Next, choose the compensation capacitor to set the compensa-
tion zero, f
Z1
, to the lesser of 1/5 of the crossover frequency or
1/2 of the LC resonant frequency.
I
Z
SWCO
Z1
CR
ff
f
2
1
505
(14)
or
I
Z
LC
Z1
CR
f
f
2
1
2
(15)
Solving for C
I
in Equation 14 yields
SW
Z
I
fR
C
25
(16)
Solving for C
I
in Equation 15 yields
LC
Z
I
fR
C
1
(17)
Use the larger value of C
I
from Equation 16 or Equation 17.
Because of the finite output current drive of the error amplifier,
C
I
must be less than 10 nF. If it is larger than 10 nF, choose a
larger R
TOP
and recalculate R
Z
and C
I
until C
I
is less than 10 nF.
Next, choose the high frequency pole, f
P1
, to be 1/2 of f
SW
.
SW
P1
ff
2
1
(18)
Because C
HF
<< C
I
,
HF
Z
P1
CR
f
2
1
(19)
Combine Equation 18 and Equation 19, and solve for C
HF
.
Z
SW
HF
Rf
C
1
(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, R
SNUB
is typically 2 Ω to 4 Ω, and C
SNUB
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 R
SNUB
is
SWSNUB
IN
SNUB
fCVP
2
In most applications, a component size of 0805 for R
SNUB
is
sufficient. The RC snubber does not reduce the voltage over-
shoot. A resistor, R
RISE
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, R
RISE
is not needed.
Figure 29. Application Circuit with a Snubber
G
(dB)
PHASE
–180°
–270°
f
Z
f
P
C
HF
C
I
R
Z
R
TOP
R
BOT
V
OUT
INTERNAL
V
REF
EA
FB
COMP
–
1
S
L
O
P
E
–
1
S
L
O
P
E
10595-028
V
IN
ADP1851
R
RISE
M1
M2
L
V
OUT
C
SNUB
C
OUT
R
SNUB
DH
DL
SW
BST
PGND
10595-029