Datasheet

ADP1614 Data Sheet
Rev. A | Page 16 of 20
To determine the crossover frequency, it is important to note that
at the crossover frequency the compensation impedance (Z
COMP
)
is dominated by a resistor, and the output impedance (Z
OUT
) is
dominated by the impedance of an output capacitor. Therefore,
when solving for the crossover frequency, the equation (by defi-
nition of the crossover frequency) is simplified to
1
2
1
=
×
×
π
×××
××
=
OUT
C
CSCOMP
MEA
OUT
IN
OUT
FB
VL
C
f
GRG
V
V
V
V
A
(15)
where:
R
COMP
is the compensation resistor.
f
C
is the crossover frequency.
Solve for R
COMP
as follows:
CSMEA
IN
FB
OUTOUT
C
COMP
G
GVV
VC
f
R
×
××
××
×π
=
2
)
(2
(16)
where:
V
FB
= 1.245 V.
G
MEA
= 150 µA/V.
G
CS
= 7 A/V.
Therefore,
IN
OUTOUTC
COMP
V
VCf
R
2
)(4806 ×××
=
(17)
After the compensation resistor is known, set the zero formed
by the compensation capacitor and resistor to one-fourth of the
crossover frequency, or
COMPC
COMP
Rf
C
××π
=
2
(18)
where C
COMP
is the compensation capacitor.
R
COMP
C
COMP
C2
1
COMP
g
m
ERROR
AMPLIFIER
2
FB
V
BG
10293-026
Figure 34. Compensation Components
Capacitor C2 is chosen to cancel the zero introduced by the ESR
of the output capacitor.
Solve for C2 as follows:
COMP
OUT
R
CESR
C2
×
=
(19)
If a low ESR, ceramic output capacitor is used for C
OUT
, C2 is
optional. For optimal transient performance, R
COMP
and C
COMP
might need to be adjusted by observing the load transient response
of the ADP1614. For most applications, the compensation resistor
should be within the range of 1 kΩ to 100 kΩ, and the compen-
sation capacitor should be within the range of 100 pF to 10 nF.
SOFT START CAPACITOR
Upon startup (EN ≥ 1.6 V) or fault recovery, the voltage at SS
ramps up slowly by charging the soft start capacitor (C
SS
) with
an internal 5.5 µA current source (I
SS
). As the soft start
capacitor charges, it limits the peak current allowed by the part
to prevent excessive overshoot at startup. Use the following
equation to determine the necessary value of the soft start
capacitor (C
SS
) for a specific overshoot and start-up time when
the part is at the current limit with maximum load:
SS
SSSS
V
t
IC
∆
=
(20)
where:
I
SS
= 5.5 μA (typical).
Δt is the start-up time at the current limit.
V
SS
= 1.23 V (typical).
If the applied load does not place the part at the current limit,
the value of C
SS
can be reduced. A 68 nF soft start capacitor
results in negligible input current overshoot at startup and,
therefore, is suitable for most applications. If an unusually large
output capacitor is used, a longer soft start period is required to
prevent input inrush current.
However, if fast startup is required, the soft start capacitor can
be reduced or removed, which allows the ADP1614 to start
quickly but with greater peak switch current.