Datasheet
EVAL-ADP1621
Rev. 0 | Page 5 of 12
Current Limit
The current limit in the ADP1621 limits the peak inductor
current and is achieved by the COMP voltage clamp. The peak
inductor current, I
L,PK
, is given by
CS
SWMINOFF
SPKSCZCTCOMP
CLAMPCOMP
PKL
R
ft
DRI
n
V
V
I
×−
×
×
−
−
=
,
,,
,
,
1
(17)
where:
V
COMP,CLAMP
is the COMP clamp voltage (maximum 2.1 V).
V
COMP,ZCT
is the COMP zero-current threshold (typically 1.0 V).
n is the current-sense amplifier gain (typically 9.5).
I
SC,PK
is the peak slope-compensation current (typically 70 μA).
R
S
is the slope-compensation resistor.
D is the duty cycle.
f
SW
is the switching frequency.
t
OFF,MIN
is the minimum off time (typically 190 ns).
R
CS
is the current-sense resistor.
The peak inductor current limit also limits the maximum load
current at a given output voltage. The maximum load current,
assuming continuous conduction mode (CCM) operation, is
given by
()
×−= DI
MAXLOAD
1
,
⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎜
⎝
⎛
××
×
−
×−
××
−
−
Lf
DV
R
ft
DRI
n
VV
SW
IN
CS
SWMINOFF
SPKSCZCTCOMPCLAMPCOMP
2
1
,
,,,
(18)
where:
n = 9.5
V
COM,CLAMP
= 2.1 V
V
COMP,ZCT
= 1.0 V.
The current limit, I
LOAD,MAX,
is calculated to be ~4 A for this
demo board. Note that the diode SSA33L is not rated for
continuous current operation at 4 A. The maximum load
current for this demo board is designed for 2 A.
Note that the current limit in the ADP1621 is a soft current
limit. When the inductor current reaches the I
L,PK
limit given in
Equation 17, the duty cycle decreases, and the output voltage
drops below the desired voltage. The I
L,PK
limit then increases in
response to the smaller duty cycle, D. The larger the slope-
compensation resistor (R
S
), the larger the effect on I
L,PK
for an
incremental decrease in D. This behavior results in a soft
current limit for the ADP1621. Use values of R
S
that are as close
as possible to the calculated limit derived from Equation 16 to
minimize the softness of the current limit while maintaining
current-loop stability. If high-precision current limiting is
required, consider inserting a fuse in series with the inductor.
Power Components
See Table 3 for the temperature of the power components after
running the board at a 2 A load for 30 minutes.
T
A
= 22°C
Table 3.
Power Component T(°C)
2.5 μH Inductor 34
MOSFET IRF7470 44
ADP1621 32