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ADM1275 Datasheet(PDF) 23 Page - Analog Devices |
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ADM1275 Datasheet(HTML) 23 Page - Analog Devices |
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23 / 48 page  Data Sheet ADM1275 Rev. D | Page 23 of 48 ADM1275 GND VCAP ISET C1 R1 R2 Figure 53. Adjustable 5 mV to 25 mV Current Sense Limit The VCAP pin has a 2.7 V (±1.5%) internal generated voltage that can be used to set a voltage at the ISET pin. Assuming that VISET equals the voltage on the ISET pin, the resistor divider should be sized to set the ISET voltage as follows: VISET = VSENSE × 50 where VSENSE is the current sense voltage limit. The VCAP rail can also be used as the pull-up supply for setting the I2C address. The VCAP pin should not be used for any other purpose. To guarantee accuracy specifications, care should be taken not to load the VCAP pin by more than 100 μA. SOFT START A capacitor connected to the SS pin determines the inrush current profile. Before the FET is enabled, the output voltage of the current limit reference selector block is clamped at 100 mV. This, in turn, holds the hot-swap sense voltage current limit, VSENSECL, at approximately 2 mV. When the FET is requested to turn on, the SS pin is held at ground until the voltage between the SENSE+ and SENSE− pins (VSENSE) reaches the circuit breaker voltage, VCB. VCB = VSENSECL − VCBOS where VCBOS is typically 0.88 mV, making VCB = 1.12 mV. When the load current generates a sense voltage equal to VCB, a 10 μA current source is enabled, which charges the SS capacitor and results in a linear ramping voltage on the SS pin. The current limit reference also ramps up accordingly, allowing the regulated load current to ramp up while avoiding sudden transients during power-up. The SS capacitor value is given by ISET SS SS V t I C where: ISS = 10 μA. t = SS ramp time. For example, a 10 nF capacitor gives a soft start time of 1 ms. Note that the SS voltage may intersect with the FLB (foldback) voltage, and the current limit reference may change to follow FLB (see Figure 51). This change has minimal impact on startup because the output voltage rises at a similar rate to the SS voltage. GATE SENSE+ ADM1275 GND SENSE– ×50 SS CURRENT LIMIT FLB ISET TIMEOUT CURRENT LIMIT CONTROL REF SELECT 1.0V CURRENT LIMIT VCAP 10µA GATE DRIVE/ LOGIC + + – – VCP Figure 54. Soft Start FOLDBACK Foldback is a method to actively reduce the current limit as the voltage drop across the FET increases. It keeps the power across the FET to a minimum during power-up, overcurrent, or short- circuit events. It also avoids the need to oversize the FET to accommodate worst-case conditions, resulting in board size and cost savings. The ADM1275 detects the voltage drop across the FET by looking at a resistor-divided version of the output voltage. It is assumed that the supply voltage remains constant and within tolerance. The device therefore relies on the principle that the drain of the FET is at the maximum expected supply voltage, and that the magnitude of the output voltage is relative to that of the VDS of the FET. Using a resistor divider from the output voltage to the FLB pin, a relationship from VOUT, and thus VDS, to VFLB can be derived. The resistor divider should be designed to output a voltage equal to ISET when VOUT falls below the desired level. This should be well below the working tolerance of the supply rail. As VOUT continues to drop, the current limit reference follows VFLB because it is now the lowest voltage input to the current limit reference selector block. This results in a reduction of the current limit and, therefore, the regulated load current. To prevent complete current flow restriction, a clamp becomes active when the current limit reference reaches 200 mV. The current limit cannot drop below this level. To suit the SOA characteristics of a particular FET, the required minimum current for this clamp varies from design to design. However, the current limit reference fixes this clamp at 200 mV, which equates to 4 mV at the sense resistor. Therefore, the main ISET voltage can be adjusted to align this clamp to the required percentage current reduction. For example, if ISET equals 0.8 V, the clamp can be set at 25% of the maximum current. |
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