Datasheet

Data Sheet ADP1612/ADP1613
Rev. D | Page 15 of 28
To determine the crossover frequency, it is important to note
that, at that 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
definition of the crossover frequency) is simplified to
1
2
1
=
××π
×××××=
OUT
C
CSCOMPMEA
OUT
IN
OUT
FB
VL
Cf
GRG
V
V
V
V
A
(15)
where:
f
C
is the crossover frequency.
R
COMP
is the compensation resistor.
Solve for R
COMP
,
CSMEA
INFB
OUTOUT
C
COMP
GGVV
VCf
R
×××
×××π
=
2
)(2
(16)
where:
V
FB
= 1.235 V.
G
MEA
= 80 µA/V.
G
CS
= 13.4 A/V.
IN
OUTOUT
C
COMP
V
VCf
R
2
)(4746 ×××
=
(17)
Once 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
06772-004
Figure 35. Compensation Components
The capacitor, C2, is chosen to cancel the zero introduced by
output capacitance, ESR.
Solve for C2 as follows:
COMP
OUT
R
CESR
C2
×
=
(19)
For low ESR output capacitance such as with a ceramic
capacitor, 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 ADP1612/ADP1613. For most
applications, the compensation resistor should be within the
range of 4.7 kΩ to 100 kΩ and the compensation capacitor
should be within the range of 100 pF to 3.3 nF.
SOFT START CAPACITOR
Upon startup (EN ≥ 1.6 V), the voltage at SS ramps up slowly
by charging the soft start capacitor (C
SS
) with an internal 5 µA
current source (I
SS
). As the soft start capacitor charges, it limits
the peak current allowed by the part to prevent excessive over-
shoot at startup. The necessary soft start capacitor, C
SS
, for a
specific overshoot and start-up time can be calculated for the
maximum load condition when the part is at current limit by:
SS
SSSS
V
t
IC
∆
=
(20)
where:
I
SS
= 5 μA (typical).
V
SS
= 1.2 V.
Δt = startup time, at current limit.
If the applied load does not place the part at current limit, the
necessary C
SS
will be smaller. A 33 nF soft start capacitor results
in negligible input current overshoot at start up, and therefore is
suitable for most applications. However, if an unusually large
output capacitor is used, a longer soft start period is required
to prevent input inrush current.
Conversely, if fast startup is a requirement, the soft start
capacitor can be reduced or removed, allowing the
ADP1612/ADP1613 to start quickly, but allowing greater
peak switch current.