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ISL6549CRZ Datasheet(PDF) 9 Page - Renesas Technology Corp |
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ISL6549CRZ Datasheet(HTML) 9 Page - Renesas Technology Corp |
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9 / 18 page  ISL6549 Rev X.00 Page 9 of 18 soft-start cycles (1 internal soft-start ramp cycle, plus one-quarter on the next). If either VINx voltage is not present at startup, that will cause a UV shutdown and restart cycle; similarly, if either VINx is removed after start-up, a shutdown and restart cycle will start when its output drifts down to the UV trip point. But in both cases, once the VINx is restored, the VOUTs will recover on the next soft-start ramp. Figure 5 shows an example of the start-up, with VIN1 not powered. VOUT2 ramps up one-quarter of the way, at which time the UV comparators are enabled. Since VIN1 is not present, VOUT1 will not be following the soft-start ramp up, and it will fail the test for UV, shutting down both outputs. It starts an internal delay time-out (equal to one soft-start interval), and then starts a new ramp. For this example, it shows about a 1.6ms ramp up, and 6.4ms off, before the next ramp starts. Thus, the total period of 8ms is based on 1.25 soft-start cycles (one-quarter of the first ramp, and then one full time-out, at a clock period of around 1.6µs) The dotted magenta line shows the case where VOUT2 is allowed to ramp all of the way up to 2V. Switching Frequency The switching frequency of the ISL6549 is determined by the value of the FS resistor. The graph in Figure 6 shows the dependence between the resistor chosen and the resulting switching frequency. Output Voltage Selection The output voltage of the PWM converter can be programmed to any level between VIN1 and the internal reference, 0.8V. However, even though the ISL6549 can run at near 100% duty cycle at zero load, additional voltage margin is required above VIN1 to allow for loading. An external resistor divider is used to scale the output voltage relative to the reference voltage and feed it back to the inverting input of the error amplifier (see Figure 7). A typical value for R1 may be 1.00k (±1% for accuracy), and then R4 (also ±1%) is chosen according to Equation 1: R1 is also part of the compensation circuit (see “PWM Controller Feedback Compensation” on page 10 for more details), so once chosen for that, it should not be changed to adjust VOUT1; only change R4. If the output voltage desired is 0.8V, simply route VOUT1 back to the FB pin through R1, but do not populate R4. VOUT1 voltages less than the 0.8V reference are not available. The linear regulator output voltage is also set by means of an external resistor divider as shown in Figure 8. Select a value for R5 (typical 1.00k ±1% for accuracy), and use Equation 2 to calculate R6 (also ±1%), where VOUT2 is the desired linear regulator output voltage and VREF is the internal reference voltage, 0.8V. For an output voltage of 0.8V, simply populate R5 with a value less than 5k and do not populate R6. VOUT2 voltages less than the 0.8V reference are not available. FIGURE 5. UNDERVOLTAGE PROTECTION (SIMULATED BY HAVING NO VIN1 ON POWER-UP) GND> VOUT2 (0.5V/DIV) VOUT2 (0.5V/DIV) GND> 1.6ms 6.4ms VOUT1 (0.5V/DIV) 100k 1M 10k 100k 1M R (k ) FIGURE 6. FREQUENCY vs FS RESISTOR R4 R1 0.8V VOUT1 0.8V – ---------------------------------------- = (EQ. 1) FIGURE 7. OUTPUT VOLTAGE SELECTION OF THE SWITCHER (VOUT1) R1 COUT1 VOUT1 R4 LOUT ISL6549 Q1 FB UGATE COMP R2 C1 C2 R3 C3 Q2 LGATE VIN1 + CIN1 + VOUT1 0.8 1 R1 R4 -------- + = PHASE R6 R5 0.8V VOUT2 0.8V – --------------------------------------- = (EQ. 2) |
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