LTC4210-3CS6 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4210-3CS6 Datasheet PDF : 16 Pages
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LTC4210-3/LTC4210-4
APPLICATIO S I FOR ATIO
GATE ramp up. If the RSENSE current is below the current
limit, the GATE ramps at a constant rate of:
∆VGATE = IGATE
∆T CGATE
(6)
where CGATE is the total capacitance at the GATE pin.
The current through RSENSE can be divided into two
components; ICLOAD due to the total load capacitance
(CLOAD) and ILOAD due to the noncapacitive load elements.
The capacitive load typically dominates.
For a successful start-up without current limit, IRSENSE <
ILIMIT:
IRSENSE = ICLOAD + ILOAD < ILIMIT
IRSENSE
=
⎛
⎝⎜
CLOAD
•
∆VOUT
∆T
⎞
⎠⎟
+
ILOAD
< ILIMIT
(7)
Due to the voltage follower configuration, the VOUT ramp
rate approximately tracks VGATE:
∆VOUT
∆T
=
ICLOAD
CLOAD
≈
∆VGATE
∆T
=
IGATE
CGATE
(8)
At time point 6, VOUT is approximately VIN 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.
Gate Start-Up Time
The start-up time without current limit is given by:
tSTARTUP
=
CGATE
•
VTH + VIN
IGATE
(10)
tSTARTUP
=
CGATE
•
VTH
IGATE
+ CGATE
•
VIN
IGATE
12
>2.5V
VIN
3 4 5 5A 5B 6
7
>1.3V
VON
VTIMER
VGATE
VOUT
IRSENSE
RESET
MODE
COMP2
100µA 60µA 2µA
COMP1
100µA
5µA
10µA
<10µA
10µA
VTH
DISCHARGE
BY LOAD
REGULATED AT 50mV/RSENSE
INITIAL
CYCLE
START-UP
CYCLE
NORMAL
CYCLE
4210 F04
Figure 4. Operating Sequence with
Current Limiting at Start-Up Cycle
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 IRSENSE by decreasing the IGATE current (<10µA).
IRSENSE
=
ILIMIT
=
50mV
RSENSE
(9)
Equations 7 and 8 are applicable but with a lower GATE and
VOUT ramp rate.
During current limiting, the second term in Equation 10 is
partly modified from CGATE • VIN/IGATE to CLOAD •
VIN/ICLOAD. The start-up time is now given by:
tSTARTUP
= CGATE
• VTH
IGATE
+ CLOAD
•
VIN
ICLOAD
(11)
= CGATE
• VTH
IGATE
+ CLOAD
•
VIN
IRSENSE – ILOAD
For successful completion of current limit start-up cycle
there must be a net current to charge CLOAD and the
current limit duration must be less than tCBDELAY. The
second term in Equation 11 has to fulfill Equation 12.
421034fa
11
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