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EL7532IY-T7 Datasheet(PDF) 6 Page - Intersil Corporation |
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EL7532IY-T7 Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 8 page 6 FN7435.7 December 8, 2006 Applications Information Product Description The EL7532 is a synchronous, integrated FET 2A step-down regulator which operates from an input of 2.5V to 6V. The output voltage is user-adjustable with a pair of external resistors. The internally-compensated controller makes it possible to use only two ceramic capacitors and one inductor to form a complete, very small footprint 2A DC-DC converter. 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 output voltage is gradually increased to ensure proper soft- start operation. When the EN pin is connected to a logic low, the EL7532 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. PWM Operation In the PWM mode, the P channel MOSFET and N channel MOSFET always operate complementary. When the PMOSFET is on and the NMOSFET off, the inductor current increases linearly. The input energy is transferred to the output and also stored in the inductor. When the P channel MOSFET is off and the N channel MOSFET on, the inductor current decreases linearly, and energy is transferred from the inductor to the output. Hence, the average current through the inductor is the output current. Since the inductor and the output capacitor act as a low pass filter, the duty cycle ratio is approximately equal to VO divided by VIN. The output LC filter has a second order effect. To maintain the stability of the converter, the overall controller must be compensated. This is done with the fixed internally compensated error amplifier and the PWM compensator. Because the compensations are fixed, the values of input and output capacitors are 10µF to 22µF ceramic. The inductor is nominally 1.8µH, though 1.5µH to 2.2µH can be used. 100% Duty Ratio Operation EL7532 utilizes CMOS power FET's as the internal synchronous power switches. The upper switch is a PMOS and lower switch a NMOS. This not only saves a boot capacitor, it also allows 100% turn-on of the upper PFET switch, achieving VO close to VIN. The maximum achievable VO is, Where RL is the DC resistance on the inductor and RDSON1 the PFET on-resistance, nominal 70m Ω at room temperature with tempco of 0.2m Ω/°C. As the input voltage drops gradually close or even below the preset VO, the converter gets into 100% duty ratio. At this condition, the upper PFET needs some minimum turn-off time if it is turned off. This off-time is related to input/output conditions. This makes the duty ratio appear randomly and increases the output ripple somewhat until the 100% duty ratio is reached. A larger output capacitor could reduce the random-looking ripple. Users need to verify if this condition has an adverse effect on the overall circuit if close to 100% duty ratio is expected. RSI/POR Function When powering up, the open-collector Power-On-Reset output holds low for about 100ms after VO reaches the preset voltage. When the active-HI reset signal RSI is issued, POR goes to low immediately and holds for the same period of time after RSI comes back to LOW. The output voltage is unaffected. (Please refer to the timing diagram). When the function is not used, connect RSI to ground and leave open the pull-up resister R4 at POR pin. The POR output also serves as a 100ms delayed Power Good signal when the pull-up resister R4 is installed. The RSI pin needs to be directly (or indirectly through a resister R5) connected to Ground for this to function properly. Output Voltage Selection Users can set the output voltage of the converter 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. We recommend 10µF to 22µF multi-layer ceramic capacitors with X5R or X7R rating for both the input and output capacitors, and 1.5µH to 2.2µH inductance for the inductor. V O V IN R L R DSON1 + () I O × – = 100ms MIN 25ns 100ms POR RSI VO FIGURE 13. RSI & POR TIMING DIAGRAM V O 0.8 1 R 2 R 1 ------- + ⎝⎠ ⎜⎟ ⎛⎞ × = EL7532 |
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