Datasheet

12
LTC4210-1/LTC4210-2
421012f
The current through R
SENSE
can be divided into two
components; I
CLOAD
due to the total load capacitance
(C
LOAD
) and I
LOAD
due to the noncapacitive load elements.
The capacitive load typically dominates.
For a successful start-up without current limit, I
RSENSE
<
I
LIMIT
:
I
RSENSE
= I
CLOAD
+ I
LOAD
< I
LIMIT
IC
V
T
II
RSENSE LOAD
OUT
LOAD LIMIT
=
∆
∆






+<•
(7)
Due to the voltage follower configuration, the V
OUT
ramp
rate approximately tracks V
GATE
:
∆
∆
=≈
∆
∆
=
V
T
I
C
V
T
I
C
OUT CLOAD
LOAD
GATE GATE
GATE
(8)
At time point 6, V
OUT
is approximately V
IN
but GATE ramp-
up continues until it reaches a maximum voltage. This
maximum voltage is determined either by the charge
pump or the internal clamp.
Start-Up Cycle With Current Limit
If the duration of the current limit is brief during start-up
(Figure 4) and it did not last beyond the circuit breaker
function time out, the GATE behaves the same as in start-
up without current limit except for the time interval be-
tween time point 5A and time point 5B. The servo amplifier
limits I
RSENSE
by decreasing the I
GATE
current (<10µA).
II
mV
R
RSENSE LIMIT
SENSE
==
50
(9)
Equations 8 and 9 are applicable but with a lower GATE and
V
OUT
ramp rate.
Gate Start-Up Time
The start-up time without current limit is given by:
tC
VV
I
tC
V
I
C
V
I
STARTUP GATE
TH IN
GATE
STARTUP GATE
TH
GATE
GATE
IN
GATE
=
+
=+
•
••
(10)
During current limiting, the second term in Equation 10 is
partly modified from C
GATE
• V
IN
/I
GATE
to C
LOAD
•
V
IN
/I
CLOAD
. The start-up time is now given by:
tC
V
I
C
V
I
C
V
I
C
V
II
STARTUP GATE
TH
GATE
LOAD
IN
CLOAD
GATE
TH
GATE
LOAD
IN
RSENSE LOAD
=+
=+
••
••
–
For successful completion of current limit start-up cycle
there must be a net current to charge C
LOAD
and the
current limit duration must be less than t
CBDELAY
. The
second term in equation 11 has to fulfill equation 12.
C
V
II
t
LOAD
IN
RSENSE LOAD
CBDELAY
•
–
<
(12)
APPLICATIO S I FOR ATIO
WUUU
V
TIMER
V
GATE
V
OUT
I
RSENSE
V
ON
V
IN
>2.5V
12 345 5A5B6 7
RESET
MODE
INITIAL
CYCLE
START-UP
CYCLE
NORMAL
CYCLE
5µA
100µA
COMP1
10µA
10µA
60µA
100µA
DISCHARGE
BY LOAD
REGULATED AT 50mV/R
SENSE
V
TH
COMP2
2µA
<10µA
4210 F04
>1.3V
Figure 4. Operating Sequence with
Current Limiting at Start-Up Cycle
(11)