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LT3509 Datasheet(PDF) 18 Page - Linear Technology |
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LT3509 Datasheet(HTML) 18 Page - Linear Technology |
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18 / 24 page  LT3509 18 3509f Diode Selection The catch diode (D1 from Figure 1) conducts current only during switch off time. Average forward current in normal operation can be calculated from: II V V V D AVG OUT IN OUT IN () (– ) / = The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current limit. If transient input voltages exceed 40V, use a Schottky diode with a reverse voltage rating of 45V or higher. If the maximum transient input voltage is under 40V, use a Schottky diode with a reverse voltage rating greater than the maximum input voltage. Table 3 lists several Schottky diodes and their manufacturers: Table 3. Schottky Diodes MANUFACTURER/ PART NUMBER VR (V) IAVE (A) VF at 1A (mV) On Semiconductor MBRM140 40 1 550 MicroSemi UPS140 40 1 450 Diodes Inc. DFLS140L 40 1 550 1N5819HW 40 1 450 Short and Reverse Protection Provided the inductors are chosen to not go deep into their saturation region at the maximum ILIMIT current the LT3509 will tolerate a short-circuit on one or both outputs. The excess current in the inductor will be detected by the DA comparator and the frequency will reduced until the valley current is below the limit. This shouldn’t affect the other channel unless the channel that is shorted is also providing the boost supply to the BD pin. In this case the voltage drop of the other switch will increase and lower the efficiency. This could eventually cause the part to reach the thermal shutdown limit. One other important feature of the part that needs to be considered is that there is a parasitic diode in parallel with the power switch. In normal operation this is reverse biased but it could conduct if the load can be powered from an alternate source when the LT3509 has no input. This may occur in battery charging applications or in battery backup systems where a bat- tery or some other supply is diode OR-ed with one of the LT3509 regulated outputs. If the SW pin is at more than about 4V the VIN pin can attain sufficient voltage for LT3509 control circuitry to power-up to the quiescent bias level and up to 2mA could be drawn from the backup supply. This can be minimized if some discrete FETs or open drain buffers are used to pull down the RUN/SS pins. Of course the gates need to be driven from the standby or battery backed supply. If there is the possibility of a short-circuit at the input or just other parallel circuits connected to VIN it would be best to add a protection diode in series with VIN. This will also protect against a reversed input polarity. These concepts are illustrated in Figure 12. APPLICATIONS INFORMATION L1 D1 SLEEP D2 VOUT COUT CBOOST CIN VIN 3509 F12 LT3509 GND VIN RUN/SS1 RUN/SS2 BD BOOST SW DA Figure 12. Reverse Bias Protection |
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