Datasheet LT1676 (Analog Devices) - 9

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