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ISL6753 Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL6753 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 15 page ![]() 10 FN9182.1 March 10, 2005 The charging time of the ramp capacitor is For optimum performance, the maximum value of the capacitor should be limited to 10nF. The maximum DC current through the resistor should be limited to 2mA maximum. For example, if the oscillator frequency is 400kHz, the minimum input voltage is 300V, and a 4.7nF ramp capacitor is selected, the value of the resistor can be determined by rearranging EQ. 7. where t is equal to the oscillator period minus the deadtime. If the deadtime is short relative to the oscillator period, it can be ignored for this calculation. If feed forward operation is not desired, the RC network may be connected to VREF rather than the input voltage. Alternatively, a resistor divider from CTBUF may be used as the sawtooth signal. Regardless, a sawtooth waveform must be generated on RAMP as it is required for proper PWM operation. Slope Compensation Peak current-mode control requires slope compensation to improve noise immunity, particularly at lighter loads, and to prevent current loop instability, particularly for duty cycles greater than 50%. Slope compensation may be accomplished by summing an external ramp with the current feedback signal or by subtracting the external ramp from the voltage feedback error signal. Adding the external ramp to the current feedback signal is the more popular method. From the small signal current-mode model [1] it can be shown that the naturally-sampled modulator gain, Fm, without slope compensation, is where Sn is the slope of the sawtooth signal and Tsw is the duration of the half-cycle. When an external ramp is added, the modulator gain becomes where Se is slope of the external ramp and The criteria for determining the correct amount of external ramp can be determined by appropriately setting the damping factor of the double-pole located at half the oscillator frequency. The double-pole will be critically damped if the Q-factor is set to 1, and over-damped for Q > 1, and under-damped for Q < 1. An under-damped condition can result in current loop instability. where D is the percent of on time during a half cycle. Setting Q = 1 and solving for Se yields Since Sn and Se are the on time slopes of the current ramp and the external ramp, respectively, they can be multiplied by Ton to obtain the voltage change that occurs during Ton. where Vn is the change in the current feedback signal during the on time and Ve is the voltage that must be added by the external ramp. Vn can be solved for in terms of input voltage, current transducer components, and output inductance yielding where RCS is the current sense burden resistor, NCT is the current transformer turns ratio, LO is the output inductance, VO is the output voltage, and Ns and Np are the secondary and primary turns, respectively. FIGURE 6. VOLTAGE FEED FORWARD CONTROL VIN R3 C7 GND 1 2 4 3 5 6 7 89 10 11 12 13 14 15 16 RAMP ISL6753 tR3 C7 1 V RAMP PEAK () V IN MIN () ---------------------------------------- – ln ⋅⋅ – = S (EQ. 7) R3 t – C7 1 V RAMP PEAK () V IN MIN ) () ---------------------------------------- – ln ⋅ ------------------------------------------------------------------------- 2.5 – 10 6 – ⋅ 4.7 10 9 – 1 1 300 ---------- – ln ⋅⋅ ------------------------------------------------------------ == 159 = k Ω (EQ. 8) Fm 1 SnTsw -------------------- = (EQ. 9) Fm 1 Sn Se + ()Tsw --------------------------------------- 1 m c SnTsw ---------------------------- == (EQ. 10) m c 1 Se Sn ------- + = (EQ. 11) Q 1 π m c 1D – () 0.5 – () ------------------------------------------------- = (EQ. 12 S e S n 1 π --- 0.5 + 1 1D – ------------- 1 – = (EQ. 13) V e V n 1 π --- 0.5 + 1 1D – ------------- 1 – = (EQ. 14) V e T SW V ⋅ O R CS ⋅ N CT LO ⋅ ------------------------------------------ N S N P -------- ⋅ 1 π --- D0.5 – + = V (EQ. 15) ISL6753 |
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