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ISL97536IUZ-TK Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL97536IUZ-TK Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 8 page 6 FN6279.1 November 2, 2007 Applications Information Product Description The ISL97536 is a synchronous, integrated FET 1A 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 1A DC/DC converter. PWM Operation In PWM switching mode, the P-Channel MOSFET and N-Channel MOSFET always operate complementary. When the P-Channel MOSFET is on and the N-Channel MOSFET 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 40µF ceramic and inductor is 1.5µH to 2.2µH. Start-Up and Shutdown 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 ISL97536 is in the shutdown 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 1.5A 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 as load demand increases. When the output voltage drops 30mV below the reference voltage, the converter will shutdown for a period of time, approximated by Equation 1, and then restart. If the overcurrent condition still exists, it will repeat the shutdown-wait-restart event. This is called a “hiccup” event. The average power dissipation is reduced, thereby reducing the likelihood of damage current and thermal conditions in the IC as shown in Equation 1. Thermal Shutdown 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 cool down. Once the junction temperature drops to about +130°C, the regulator will perform a normal restart. Thermal Performance The ISL97536 is available in a fused-lead MSOP10. Compared with regular MSOP10 package, the fused-lead package provides lower thermal resistance. The θ JA 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. 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 resistor R4 at POR pin. The POR output also serves as a 100ms delayed Power Good signal when the pull-up resistor R4 is installed. The RSI pin needs to be directly (or indirectly through a resistor R6) connected to Ground for this to function properly. Output Voltage Selection Users can set the output voltage of the variable version with a resistor divider, which can be chosen based on Equation 2: tHICCUP 700 μ V IN ⋅ 3 ---------------------------- 216 μ + ⎝⎠ ⎛⎞ ≈ (EQ. 1) 100ms MIN 25ns 100ms POR RSI VO FIGURE 13. RSI AND POR TIMING DIAGRAM V O 0.8 1 R 1 R 2 ------- + ⎝⎠ ⎜⎟ ⎛⎞ × = (EQ. 2) ISL97536 |
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