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LT1676I Datasheet(PDF) 9 Page - Linear Technology |
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LT1676I Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1676 which tantalum capacitors are generally unavailable. Rela- tively bulky “high frequency” aluminum electrolytic types, specifically constructed and rated for switching supply applications, may be the only choice. Minimum Load Considerations As discussed previously, a lightly loaded LT1676 with VC pin control voltage below the boost threshold will operate in low dV/dt mode. This affords greater controllability at light loads, as minimum tON requirements are relaxed. In many applications, it is possible to operate the LT1676 down to zero external load without “pulse skipping”! In these cases, the LT1676’s modest VCC current requirement of several milliamperes provides enough of a load to avoid pulse skipping. However, some users may be indifferent to pulse skipping behavior, but instead may be concerned with maintaining maximum possible efficiency at light loads. This require- ment can be satisfied by forcing the part into Burst Mode TM operation. The use of an external comparator whose output controls the shutdown pin allows high efficiency at light loads through Burst Mode operation behavior (see Typical Applications and Figure 8). Maximum Load/Short-Circuit Considerations The LT1676 is a current mode controller. It uses the VC node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached. The internal clamp on the VC node, nominally 2V, then acts as an output switch peak current limit. This action becomes the switch current limit specification. The maximum available output power is then determined by the switch current limit. A potential controllability problem could occur under short-circuit conditions. If the power supply output is short circuited, the feedback amplifier responds to the low output voltage by raising the control voltage, VC, to its peak current limit value. Ideally, the output switch would be turned on, and then turned off as its current exceeded the value indicated by VC. However, there is finite response time involved in both the current comparator and turnoff of the output switch. These result in a minimum on time APPLICATIONS INFORMATION tON(MIN). When combined with the large ratio of VIN to (VF + I • R), the diode forward voltage plus inductor I • R voltage drop, the potential exists for a loss of control. Expressed mathematically the requirement to maintain control is: ft VI R V ON F IN • • ≤ + where: f = switching frequency tON = switch ON time VF = diode forward voltage VIN = Input voltage I • R = inductor I • R voltage drop If this condition is not observed, the current will not be limited at IPK, but will cycle-by-cycle ratchet up to some higher value. Using the nominal LT1676 clock frequency of 100KHz, a VIN of 48V and a (VF + I • R) of say 0.7V, the maximum tON to maintain control would be approximately 140ns, an unacceptably short time. The solution to this dilemma is to slow down the oscillator when the FB pin voltage is abnormally low thereby indicat- ing some sort of short-circuit condition. Figure 2 shows the typical response of Oscillator Frequency vs FB divider Thevenin voltage and impedance. Oscillator frequency is unaffected until FB voltage drops to about 2/3 of its normal value. Below this point the oscillator frequency decreases roughly linearly down to a limit of about 25kHz. This lower Burst Mode is a trademark of Linear Technology Corporation. FB DIVIDER THEVENIN VOLTAGE (V) 0 0 20 40 60 80 100 120 0.25 0.50 0.75 1.00 1676 F02 1.25 RTH LT1676 FB RTH = 22k RTH = 4.7k RTH = 10k Figure 2. Oscillator Frequency vs FB Divider Thevenin Voltage and Impedance |
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