Datasheet LT3472 (Analog Devices) - 6

LT3472
U U W U APPLICATIO S I FOR ATIO Operation Inductor Selection
The LT3472 uses a constant frequency, current mode A 22µH inductor is recommended for LT3472 step-up control scheme to provide excellent line and load regula- channel. The inverter channel can use a 22µH or 47µH tion. Operation can be best understood by referring to the inductor. 47µH inductors will provide slightly more cur- block diagram in Figure 1. At the start of each oscillator rent. Small size and high efficiency are the major concerns cycle, the SR latch X1 is set, which turns on the power for most LT3472 applications. Inductors with low core switch Q1. A voltage proportional to the switch current is losses and small DCR (copper wire resistance) at 1.1MHz added to a stabilizing ramp and the resulting sum is fed are good choices for LT3472 applications. Some induc- into the positive terminal of the PWM comparator A2. tors in this category with small size are listed in When this voltage exceeds the level at the negative input Table 1. The efficiency comparison of different inductors of A2, the SR latch X1 is reset turning off the power switch is shown in Figure 3. Q1. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the 85 INVERTER LOAD = 20mA difference between the feedback voltage and the reference LQH32CN220 voltage of 1.25V. In this manner, the error amplifier sets 80 the correct peak current level to keep the output in regu- TOKO 1067FB-220M 75 lation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less LQH2MCN220 70 current is delivered. The second channel is an inverting EFFICIENCY (%) converter. The basic operation is the same as the positive 65 channel. The SR latch X2 is also set at the start of each oscillator cycle. The power switch Q2 is turned on at the 60 same time as Q1. The turn off of Q2 is determined by its 0 5 10 15 20 25 30 LOAD CURRENT IO1 (mA) own feedback loop, which consists of error amplifier A3 3473 F02a 3472 F02a and PWM comparator A4. The reference voltage of this negative channel is ground. 85 BOOST LOAD = 20mA Switching waveforms with typical load conditions are LQH32CN220 shown in Figure 2. 80 TOKO 1067FB-220M 75 VSWP LQH2MCN220 20V/DIV I 70 L1 EFFICIENCY (%) 100mA/DIV VSWN 20V/DIV 65 ISWN 100mA/DIV 60 0 5 10 15 20 25 30 3472 FO4 LOAD CURRENT I V 500ns/DIV O2 (mA) IN = 3.6V VPOS = 15V, 20mA 3473 F02a 3472 F02b VNEG = –7.5V, 30mA
Figure 2. Switching Waveforms Figure 3. Efficiency Comparison of Different Inductors
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