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ML4818CP Datasheet(PDF) 6 Page - Micro Linear Corporation |
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ML4818CP Datasheet(HTML) 6 Page - Micro Linear Corporation |
6 / 12 page 6 ML4818 The ML4818 can also be used in current mode by sensing load current on the RAMP input (pin 3). The four output delay timers are programmed via an external RDELAY resistor as shown below. This resistor value should be no less than 1k Ω. Expressing RDELAY in k Ω the delay, in ns is: TR DELAY DELAY =× + 33 45 (1) The ML4818 contains special logic circuits to provide for voltage mode feed-forward and lock out long pulses into the internal logic. This prevents instability from occuring when the Φ Comparator trips in voltage mode. 3 RAMP S R Q Φ MOD OUTPUT OSC QR Figure 3. Voltage Feed-Forward Circuit. The collector of QR in figure 3 is high only during a power cycle. When the power cycle terminates, RAMP is pulled low. In voltage mode operation, a capacitor is connected from RAMP to GND with a resistor from RAMP to VIN to provide input voltage feed forward. OSCILLATOR The ML4818 oscillator charges the external capacitor, CT, with a current (ISET) equal to 5/RT. When the CT voltage reaches the upper threshold (Ramp Peak), the comparator changes state, turning on the current sink which discharges CT to the lower threshold (Ramp Valley). The CT pin is clamped to Ramp Valley by Q1 (Figure 5) to prevent inaccuracy due to undershoot on CT. To use the CLOCK output for driving external synch- ronization circuitry, a pull-down resistor is required from CLOCK to GND. 11 + – 2 13 5V CLOCK OUT 5V 5.5mA 1.7V RT ISET CT ISET 250 Ω Q1 Figure 5. Ocillator Block Diagram For frequencies of less than 500kHz, oscillator frequency can be set by using the following formulae: f CR C OSC TT T = + 1 0 52 500 . (2) ERROR AMPLIFIER The ML4818 error amplifier is a 2.5MHz bandwidth, 8.5V/ µs slew rate op-amp with provision for limiting the positive output voltage swing (output inhibit line) to implement the soft start function. The error amplifier output source current is limited to 4.5mA. CLOCK TD CT RAMP PEAK RAMP VALLEY TC Figure 4. Ocillator Timing Diagram |
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