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LTC4217CFE-TR Datasheet(PDF) 11 Page - Linear Technology |
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LTC4217CFE-TR Datasheet(HTML) 11 Page - Linear Technology |
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11 / 18 page  LTC4217 11 4217fc APPLICATIONS INFORMATION Tying the TIMER pin to INTVCC will force the part to use the internally generated (circuit breaker) delay of 2ms. In either case the FLT pin is pulled low to indicate an overcurrent fault has turned off the pass MOSFET. For a given the circuit breaker time delay, the equation for setting the timing capacitor’s value is as follows: CT = tCB • 0.083(μF/ms) After the switch is turned off, the TIMER pin begins dis- charging the timing capacitor with a 2μA pull-down current. When the TIMER pin reaches its 0.2V threshold, an internal 100ms timer is started. After the 100ms delay, the switch is allowed to turn on again if the overcurrent fault has been cleared. Bringing the UV pin below 0.6V and then high will clear the fault. If the TIMER pin is tied to INTVCC then the switch is allowed to turn on again (after an internal 100ms delay) if the overcurrent fault is cleared. Tying the FLT pin to the UV pin allows the part to self- clear the fault and turn the MOSFET on as soon as TIMER pin has ramped below 0.2V. In this auto-retry mode the LTC4217 repeatedly tries to turn on after an overcurrent at a period determined by the capacitor on the TIMER pin. The auto-retry mode also functions when the TIMER pin is tied to INTVCC. The waveform in Figure 4 shows how the output latches off following a short-circuit. The current in the MOSFET is 0.5A as the timer ramps up. Current Limit Adjustment The default value of the active current limit is 2A. The current limit threshold can be adjusted lower by placing (OV pin), overcurrent circuit breaker (SENSE pin) or over temperature. Normally the switch is turned off with a 250μA current pulling down the GATE pin to ground. With the switch turned off, the OUT voltage drops which pulls the FB pin below its threshold. PG then pulls low to indicate output power is no longer good. If VDD drops below 2.65V for greater than 5μs or INTVCC drops below 2.5V for greater than 1μs, a fast shutdown of the switch is initiated. The GATE is pulled down with a 170mA current to the OUT pin. Overcurrent Fault The LTC4217 features an adjustable current limit with foldback that protects against short-circuits or excessive load current. To prevent excessive power dissipation in the switch during active current limit, the available current is reduced as a function of the output voltage sensed by the FB pin. A graph in the Typical Performance Characteristics curves shows the current limit versus FB voltage. An overcurrent fault occurs when the current limit circuitry has been engaged for longer than the timeout delay set by the TIMER. Current limiting begins when the MOSFET current reaches 0.5A to 2A (depending on the foldback). The GATE pin is then brought down with a 140mA GATE- to-OUT current. The voltage on the GATE is regulated in order to limit the current to less than 2A. At this point, a circuit breaker time delay starts by charging the external timing capacitor from the TIMER pin with a 100μA pull- up current. If the TIMER pin reaches its 1.2V threshold, the internal switch turns off (with a 250μA current from GATE to ground). Included in the Typical Performance Characteristics curves is a graph of the Safe Operating Area for the MOSFET. From this graph one can determine the MOSFET’s maximum time in current limit for a given output power. Figure 3. Compensation for Small CLOAD 4217 F03 LTC4217 *OPTIONAL RC TO LOWER INRUSH CURRENT GATE CP 2.2nF Figure 4. Short-Circuit Waveform IOUT 1A/DIV VOUT 10V/DIV ΔVGATE 10V/DIV TIMER 2V/DIV 1ms/DIV 4217 F04 |
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