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LTC1645 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1645 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  8 LTC1645 1645fa Power Supply Ramping The power supplies on a board are controlled by placing external N-channel pass transistors in the power paths as shown in Figure 2. Consult Table 1 for a selection of N-channel FETs suitable for use with the LTC1645. RSENSE1 and RSENSE2 provide current fault detection and R1 and R2 prevent high frequency oscillation. By ramping the gates of the pass transistors up and down at a controlled rate, the transient surge current (I = C • dv/dt) drawn from the main backplane supply is limited to a safe value when the board makes connection. When power is first applied to the chip, the gates of the N-channels (GATE1 and GATE2 pins) are pulled low. After the ON pin is held above 0.8V for at least one timing cycle, the voltage at GATE1 begins to rise with a slope equal to dv/dt = 10 µA/C1 (Figure 3), where C1 is the external capacitor connected between the GATE1 pin and GND. If the ON pin is brought above 2V (and the ON pin has been held above 0.8V for at least one timing cycle), the voltage at GATE2 begins to rise with a slope equal to dv/dt = 10 µA/C2. The ramp time for each supply is t = (VCCn • Cn)/10µA. If the ON pin is pulled below 2V for GATE2 or 0.8V for GATE1 (but above 0.4V), a 40 µAcurrentsourceisconnectedfrom GATE n to GND, and the voltage at the GATEn pin will ramp down, as shown in Figure 4. Ringing Good engineering practice calls for bypassing the supply rail of any circuit. Bypass capacitors are often placed at the supply connection of every active device, in addition to one or more large value bulk bypass capacitors per supply rail. If power is connected abruptly, the bypass capacitors slow the rate of rise of voltage and heavily damp any parasitic resonance of lead or trace inductance working against the supply bypass capacitors. The opposite is true for LTC1645 Hot Swap circuits on a daughterboard. In most cases, on the powered side of the N-channel FET switches (VCCn) there is no supply bypass capacitor present. An abrupt connection, produced by plugging a board into a backplane connector, results in a fast rising edge applied to the VCCn line of the LTC1645. APPLICATIO S I FOR ATIO Figure 4. Supply Turning Off Figure 3. Supply Turning On Figure 2. Typical Hot Swap Connection RSENSE2 Q1 Q2 R2 10 Ω C2 CTIMER CLOAD1 CLOAD2 R1 10 Ω C1 12 1 2 3 8 9 6 5 11 7 13 14 10 4 VCC1 ON FAULT SENSE1 GATE2 VCC2 VCC2 VCC1 VOUT2 VOUT1 LTC1645 (14-LEAD) TIMER GND COMP+ COMPOUT FB SENSE2 GATE1 1645 F02 RESET + + RSENSE1 VCCn VOUTn VCCn + ∆VGATE 1645 F03 t1 t2 GATE n SLOPE = 10 µA/Cn VCCn VOUTn VCCn + ∆VGATE 1645 F04 t3 t4 GATE n SLOPE = 40 µA/Cn |
Similar Part No. - LTC1645_15 |
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Similar Description - LTC1645_15 |
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