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LM34919 Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM34919 Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 18 page Regulation Comparator The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch stays on for the pro- grammed on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch stays off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 100 nA over temperature. Over-Voltage Comparator The voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time pulse is im- mediately terminated. This condition can occur if the input voltage or the output load changes suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V. ON-Time Timer, and Shutdown The on-time is determined by the R ON resistor and the input voltage (V IN), and is calculated from: (4) The inverse relationship with V IN results in a nearly constant frequency as V IN is varied. To set a specific continuous con- duction mode switching frequency (F S), the RON resistor is determined from the following: (5) In high frequency applicatons the minimum value for t ON is limited by the maximum duty cycle required for regulation and the minimum off-time of (155 ns, ±15%). The minimum off- time limits the maximum duty cycle achievable with a low voltage at V IN. At high values of VIN, the minimum on-time is limited to ≊ 120 ns. The LM34919 can be remotely shut down by taking the RON/ SD pin below 0.8V. See Figure 3. In this mode the SS pin is internally grounded, the on-timer is disabled, and bias cur- rents are reduced. Releasing the RON/SD pin allows normal operation to resume. The voltage at the RON/SD pin is be- tween 1.4V and 4.0V, depending on V IN and the RON resistor. 30004413 FIGURE 3. Shutdown Implementation Current Limit Current limit detection occurs during the off-time by monitor- ing the recirculating current through the free-wheeling diode (D1). Referring to the Block Diagram, when the buck switch is turned off the inductor current flows through the load, into SGND, through the sense resistor, out of ISEN and through D1. If that current exceeds 0.64A the current limit comparator output switches to delay the start of the next on-time period. The next on-time starts when the current out of ISEN is below 0.64A and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 0.64A during each on-time, that is detected at the beginning of each off-time. The operating frequency is lower due to longer-than-normal off-times. Figure 4 illustrates the inductor current waveform. During nor- mal operation the load current is Io, the average of the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches 0.64A. During the Current Limited portion of Figure 4, the current ramps down to 0.64A during each off-time, initiating the next on-time (as- suming the voltage at FB is <2.5V). During each on-time the current ramps up an amount equal to: ΔI = (V IN - VOUT) x tON / L1 During this time the LM34919 is in a constant current mode, with an average load current (I OCL) equal to 0.64A + ΔI/2. Generally, in applications where the switching frequency is higher than ≊300 kHz and uses a small value inductor, the higher dl/dt of the inductor's ripple current results in an effec- tively lower valley current limit threshold due to the response time of the current limit detection circuit. However, since the small value inductor results in a relatively high ripple current amplitude ( ΔI in Figure 4), the load current (I OCL) at current limit is typically in excess of 640 mA. www.national.com 10 |
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Podobny opis - LM34919 |
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