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ISL8844AMUZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL8844AMUZ Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 15 page 10 FN6320.3 April 18, 2007 Pin Descriptions RTCT - This is the oscillator timing control pin. The operational frequency and maximum duty cycle are set by connecting a resistor, RT, between VREF and this pin and a timing capacitor, CT, from this pin to GND. The oscillator produces a sawtooth waveform with a programmable frequency range up to 2.0MHz. The charge time, tC, the discharge time, tD, the switching frequency, f, and the maximum duty cycle, Dmax, can be approximated from the following equations: The formulae have increased error at higher frequencies due to propagation delays. Figure 4 may be used as a guideline in selecting the capacitor and resistor values required for a given frequency. COMP - COMP is the output of the error amplifier and the input of the PWM comparator. The control loop frequency compensation network is connected between the COMP and FB pins. FB - The output voltage feedback is connected to the inverting input of the error amplifier through this pin. The non-inverting input of the error amplifier is internally tied to a reference voltage. CS - This is the current sense input to the PWM comparator. The range of the input signal is nominally 0V to 1.0V and has an internal offset of 100mV. GND - GND is the power and small signal reference ground for all functions. OUT - This is the drive output to the power switching device. It is a high current output capable of driving the gate of a power MOSFET with peak currents of 1.0A. This GATE output is actively held low when VDD is below the UVLO threshold. VDD - VDD is the power connection for the device. The total supply current will depend on the load applied to OUT. Total IDD current is the sum of the operating current and the average output current. Knowing the operating frequency, f, and the MOSFET gate charge, Qg, the average output current can be calculated from: To optimize noise immunity, bypass VDD to GND with a ceramic capacitor as close to the VDD and GND pins as possible. VREF - The 5.00V reference voltage output. +1.0/-1.5% tolerance over line, load and operating temperature. Bypass to GND with a 0.1 μF to 3.3μF capacitor to filter this output as needed. Functional Description Features The ISL884xA current mode PWM makes an ideal choice for low-cost flyback and forward topology applications. With its greatly improved performance over industry standard parts, it is the obvious choice for new designs or existing designs which require updating. Oscillator The ISL884xA has a sawtooth oscillator with a programmable frequency range to 2MHz, which can be programmed with a resistor from VREF and a capacitor to GND on the RTCT pin. (Please refer to Figure 4 for the resistor and capacitance required for a given frequency.) FIGURE 3. EA REFERENCE vs TEMPERATURE FIGURE 4. RESISTANCE FOR CT CAPACITOR VALUES GIVEN Typical Performance Curves (Continued) -60 -40 -20 0 20 40 60 80 100 120 140 0.996 0.997 0.998 1.000 1.001 TEMPERATURE (°C) 1 10 100 1 10 100 103 RT (k Ω) 100pF 220pF 330pF 470pF 1.0nF 2.2nF 3.3nF 4.7nF 6.8nF t C 0.533 RT CT ⋅⋅ ≈ (EQ. 1) t D RT – CT In 0.008 RT 3.83 – ⋅ 0.008 RT 1.71 – ⋅ ---------------------------------------------⎠ ⎞ ⎝ ⎛ ⋅⋅ ≈ (EQ. 2) f 1t C t D + () ⁄ = (EQ. 3) Dt C f ⋅ = (EQ. 4) I OUT Qg f × = (EQ. 5) ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A |
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