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LX1660 Datasheet(PDF) 12 Page - Microsemi Corporation |
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LX1660 Datasheet(HTML) 12 Page - Microsemi Corporation |
12 / 15 page A D V ANCED PWM C ONTROLLER LX1660/1661 PRODUCT DA T ABOOK 1996/1997 Copyright © 1998 Rev. 1.1 7/98 12 P RODUCTION D ATA S HEET USING THE LX1660/61 DEVICES CURRENT LIMIT (continued) Recommended sense resistor sizes are given in the following table: CURRENT LIMIT (continued) The dc/static tripping current I trip,S satisfies: I trip,S = Select L/R SCS ≤ RL to have higher dynamic tripping curr ent than the static one. The dynamic tripping current I trip,d satisfies: I trip,d = General Guidelines for Selecting R S , CS , and RL R L = Select: R S ≤ 10 kΩ and C S according to: C Sn = The above equation has taken into account the current-de- pendency of the inductance. Typical values are: R L = 3mΩ, RS = 9k Ω, C S = 0.1µF, and L is 2.5µH at 0A current. In cases where R L is so large that the trip point current would be lower than the desired short-circuit current limit, a resistor (R S2) can be put in parallel with C S, as shown in Figure 8. The selection of components is as follows: = C S = = * Again, select (R S2 //RS) < 10k Ω. C4 ERROR COMPARATOR INPUT BYPASS CAPACITOR The LX1660/61 device has a unique topology which results in extremely fast response to transient disturbances. Actual loop closure is around a comparator. A capacitor should be placed between the INV and NINV Error Comparator inputs to eliminate jitter and noise. This capacitor value should be: C = ½ C T , where C T is the timing capacitor. Refer to Capacitor C4 in the Applica- tion Information section. C7 HICCUP CAPACITOR SELECTION The hiccup capacitor controls two time periods; the ON time duration of 10% duty cycle mode, and the OFF-time duration before re-try. The ON:OFF-time ratios will always be 1:10 due to the current sources which charge (10I) and discharge (I) the hiccup capacitor. Select C HICCUP by using: Duration of reduced D operation = 100ms/µF R L L/R SCS |T(j ω)| ω 1/R SCS R L/L FIGURE 9 — Sensor Gain FIGURE 8 — Current Sense Circuit R L L R S C S V CS Current Sense Comparator Load R S2 Copper Copper Desired Resistor Dimensions (w x l) Weight Thickness Value mm inches 2 oz/ft2 68µm 2.5m Ω 2.5 x 22 0.1 x 0.85 5m Ω 2.5 x 43 0.1 x 1.7 TABLE 3 - PCB Sense Resistor Selection Guide Loss-Less Current Sensing Using Resistance of Inductor Any inductor has a parasitic resistance, R L, which causes a DC voltage drop when current flows through the inductor. Figure 8 shows a sensor circuit comprising of a surface mount resistor, R S, and capacitor, C S, in parallel with the inductor, eliminating the current sense resistor. The current flowing through the inductor is a triangle wave. If the sensor components are selected such that: L/R L = RS * C S The voltage across the capacitor will be equal to the current flowing through the resistor, i.e. V CS = I LRL Since V CS reflects the inductor current, by selecting the appro- priate R S and C S , VCS can be made to reach the comparator voltage at the desired trip current. Design Example (Pentium II circuit, with a maximum static current of 14.2A) The gain of the sensor can be characterized as: V trip L/(R SCS) V trip R L V trip I trip,S L n R L RS R L (Required) R L (Actual) R S2 R S2 + RS L R L (Actual) * (RS2 // RS) L R L (Actual) R S + RS2 R S2 * RS |
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