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LTC4210-3CS6 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4210-3CS6 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LTC4210-3/LTC4210-4 421034fa 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: ∆ ∆ = V T I C GATE GATE GATE (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 IC V T II RSENSE LOAD OUT LOAD LIMIT = ∆ ∆ ⎛ ⎝⎜ ⎞ ⎠⎟ +< • (7) Due to the voltage follower configuration, the VOUT ramp rate approximately tracks VGATE: ∆ ∆ = ≈ ∆ ∆ = V T I C V T I C OUT CLOAD LOAD GATE GATE GATE (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. 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). II mV R RSENSE LIMIT SENSE == 50 Equations 7 and 8 are applicable but with a lower GATE and VOUT ramp rate. Gate Start-Up Time The start-up time without current limit is given by: tC VV I tC V STARTUP GATE TH IN GATE STARTUP GATE = + = • • T TH GATE GATE IN GATE I C V I + • VTIMER VGATE VOUT IRSENSE VON VIN >2.5V 12 3 4 5 5A 5B 6 7 RESET MODE INITIAL CYCLE START-UP CYCLE NORMAL CYCLE 5 µA 100 µA COMP1 10 µA 10 µA 60 µA 100 µA DISCHARGE BY LOAD REGULATED AT 50mV/RSENSE VTH COMP2 2 µA <10 µA 4210 F04 >1.3V Figure 4. Operating Sequence with Current Limiting at Start-Up Cycle (9) (10) 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: tC V I C V I C STARTUP GATE TH GATE LOAD IN CLOAD GA =+ = •• T TE TH GATE LOAD IN RSENSE LOAD V I C V II •• – + (11) 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. |
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