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EL7531IY Datasheet(PDF) 9 Page - Intersil Corporation |
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EL7531IY Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 10 page 9 FN7428.6 August 5, 2005 Forced PWM Mode/SYNC Input Pulling the SYNC pin HI (>2.5V) forces the converter into PWM mode in the next switching cycle regardless of output current. The duration of the transition varies depending on the output current. Figures 22 and 23 (under two different loading conditions) show the device goes from PFM to PWM mode Start-Up and Shut-Down When the EN pin is tied to VIN, and VIN reaches approximately 2.4V, the regulator begins to switch. The inductor current limit is gradually increased to ensure proper soft-start operation. When the EN pin is connected to a logic low, the EL7531 is in the shut-down mode. All the control circuitry and both MOSFETs are off, and VOUT falls to zero. In this mode, the total input current is less than 1µA. When the EN reaches logic HI, the regulator repeats the start-up procedure, including the soft-start function. Current Limit and Short-Circuit Protection The current limit is set at about 2A for the PMOS. When a short-circuit occurs in the load, the preset current limit restricts the amount of current available to the output, which causes the output voltage to drop below the preset voltage. In the meantime, the excessive current heats up the regulator until it reaches the thermal shut-down point. Thermal Shut-Down Once the junction reaches about 145°C, the regulator shuts down. Both the P channel and the N channel MOSFETs turn off. The output voltage will drop to zero. With the output MOSFETs turned off, the regulator will soon cool down. Once the junction temperature drops to about 130°C, the regulator will restart again in the same manner as EN pin connects to logic HI. Thermal Performance The EL7531 is available in a fused-lead MSOP10 package. Compared with regular MSOP10 package, the fused- lead package provides lower thermal resistance. The θJA is 100°C/W on a 4-layer board and 125°C/W on 2-layer board. Maximizing the copper area around the pins will further improve the thermal performance. Output Voltage Selection Users can set the output voltage of the variable version with a resister divider, which can be chosen based on the following formula: Component Selection Because of the fixed internal compensation, the component choice is relatively narrow. For a regulator with fixed output voltage, only two capacitors and one inductor are required. We recommend 10µf to 22µF multi-layer ceramic capacitors with X5R or X7R rating for both the input and output capacitors, and 1.5 to 2.2µH for the inductor. The RMS current present at the input capacitor is decided by the following formula: This is about half of the output current IO for all the VO. This input capacitor must be able to handle this current. The inductor peak-to-peak ripple current is given as: L is the inductance fS the switching frequency (nominally 1.4MHz) The inductor must be able to handle IO for the RMS load current, and to assure that the inductor is reliable, it must handle the 2A surge current that can occur during a current limit condition. Layout Considerations The layout is very important for the converter to function properly. The following PC layout guidelines should be followed: 1. Separate the Power Ground ( ) and Signal Ground ( ); connect them only at one point right at the pins 2. Place the input capacitor as close to VIN and PGND pins as possible 3. Make the following PC traces as small as possible: 4. from LX pin to L 5. from CO to PGND 6. If used, connect the trace from the FB pin to R1 and R2 as close as possible 7. Maximize the copper area around the PGND pin 8. Place several via holes under the chip to additional ground plane to improve heat dissipation The demo board is a good example of layout based on this outline. Please refer to the EL7531 Application Brief. VO 0.8 1 R2 R1 ------- + × = IINRMS VO VIN VO – () × VIN ----------------------------------------------- IO × = ∆I IL VIN ( VO) VO × – LVIN fS × × -------------------------------------------- = EL7531 |
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