Datasheet LT1777 (Analog Devices) - 10

LT1777
U U W U APPLICATIONS INFORMATION
Deciding upon a value for the sense inductor involves As an example, a maximum input voltage of 36V, an output evaluating the trade-off between overall efficiency (POUT/ voltage of 5V and a main inductor value of 220µH yields a PIN) and switch current slew rate. Larger sense inductors maximum suggested sense inductor value of 3.5µH. yield lower current slew rates which offer reduced high Circuit behavior versus sense inductor value is shown in frequency RFI emissions, but at the expense of poorer the oscilloscope photos in Figure 2. The circuit and oper- efficiency. ating conditions are similar to the Typical Application on The question is “What is the allowed range of values for a the first page of this data sheet with the exception that the sense inductor in a given application?” There is really no sense inductor is allowed to assume the series of values: minimum limit to the sense inductor, i.e., its value is 0µH, 0.47µH, 1µH and 2.2µH. Figure 2a shows a close-up allowed to be zero. (In other words, the physical sense of the leading edge (turn-on) of the current waveform. inductor ceases to exist and is replaced by a short circuit.) Values of 0µH and 0.47µH are found to yield a dI/dt of This will yield the highest efficiency possible in a given about 2.2A/µs, while 1µH yields 1.4A/µs and 2.2µH yields situation. Although an explicit current slew rate no longer 0.6A/µs. Figure 2b shows the trailing edge (turn-off) of the exists, the naturally less aggressive nature of the LT1777 will often yield quieter supply operation than other stan- dard switching regulators. As far as the maximum allowable value for the sense inductor, this is dictated by the current ramp rate in the main inductor during the conventional part of the switch- ing cycle. It is generally overconservative to limit the 100mA/DIV switch current slew rate to that exhibited by the main inductor. This would potentially yield a triangular current waveform. Efficiency would be greatly reduced at little further gain in noise performance. Stated mathematically, 1777 F02a 200ns/DIV maximum slew rate in the main inductor occurs at maxi-
(a) Leading Edge
mum input voltage as: dI Max V V IN OUT = – dt LMAIN The sense inductor experiences 2VBE of applied voltage. This is perhaps 1.0V at a maximum hot condition. If we use 100mA/DIV an additional factor of two to be conservative, this yields a maximum sense inductor value as follows: 0 5 . V Max V – V IN OUT = , or 1777 F02b 200ns/DIV L L SENSE MAIN
(b) Trailing Edge
 0 5 . V  Max L = L SENSE MAIN
Figure 2. V

SW Node Current Behavior vs LSENSE Value.
Max V – V IN OUT  
LSENSE = 0
µ
H, 0.47
µ
H, 1.0
µ
H and 2.2
µ
H
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