Datasheet LT3487 (Analog Devices) - 8

LT3487
U U W U APPLICATIO S I FOR ATIO Operation
long as CAP is typically 1.2V and worst-case 1.6V (cold) above the V The LT3487 uses a constant frequency, current mode con- BAT voltage. The drive current for the output disconnect PNP is returned to the V trol scheme to provide excellent line and load regulation. BAT pin. This allows the pass transistor to turn off when the CAP voltage Operation can be best understood by referring to the Block falls to less than 1.2V above V Diagram in Figure 1. At the start of each oscillator cycle, BAT. The VBAT pin allows applications in which the power (inductors L1 and L2) the SR latch X1 is set, which turns on the power switch and internal control circuitry (V Q1. A voltage proportional to the switch current is added IN pin) are powered from different sources. to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this
Inductor Selection
voltage exceeds the level at the negative input of A2, the SR latch X1 is reset, turning off the power switch Q1. The A 10μH inductor is recommended for the LT3487 boost level at the negative input of A2 is set by the error amplifi er channel. The inverting channel can use uncoupled 15μH A1, and is simply an amplifi ed version of the difference inductors, or coupled 10μH inductors. Small size and between the feedback voltage and the reference voltage high effi ciency are the major concerns for most LT3487 of 1.23V. In this manner, the error amplifi er sets the cor- applications. Inductors with low core losses and small rect peak current level to keep the output in regulation. DCR (copper wire resistance) at 2MHz are good choices If the error amplifi er’s output increases, more current is for LT3487 applications. The inductor DCR should be on delivered to the output; if it decreases, less current is the order of half of the switch on-resistance for its chan- delivered. The second channel is an inverting converter. nel. Some inductors in this category with small size are The basic operation is the same as the positive channel. listed in Table 1. The SR latch X2 is also set at the start of each oscillator
Table 1. Recommended Inductors
cycle. The power switch Q2 is turned on at the same time
CURRENT
as Q1. Q2 turns off based on its own feedback loop, which
INDUCTANCE DCR RATING
consists of error amplifi er A3 and PWM comparator A4.
PART NUMBER (μH) (
Ω
) (mA) MANUFACTURER
The reference voltage of this negative channel is ground. DB318C-A997AS- 10 0.18 580 Toko 100M www. tokoam.com Voltage clamps on VCP and VCN (not shown) enforce current CDRH3D18-100 10 0.205 900 Sumida limit. Switching waveforms with typical load conditions CDRH2D18HP-100 10 0.245 850 www.sumida.com are shown in Figure 2. CDRH3D23-100 10 0.117 850 CDRH2D18/HP-150 15 0.345 700 The PNP Q3 is used as an output disconnect pass transistor. CDRH3D18-150 15 0.301 750 CDRH3D23-150 15 0.191 700 Q3 disconnects the load from the input during shutdown. The anti-sat driver keeps Q3 at the edge of saturation as
Capacitor Selection
The small size of ceramic capacitors makes them suitable VSWP 20V/DIV for LT3487 applications. X5R and X7R types of ceramic ILI capacitors are recommended because they retain their 100mA/DIV V capacitance over wider voltage and temperature ranges SWN 20V/DIV than other types such as Y5V or Z5U. A 1μF input capaci- ISWN tor is suffi cient for most LT3487 applications. The output 100mA/DIV capacitors required for stability depend on the application. 200ns/DIV 3487 F02 VIN = 3.6V For the typical Li-Ion to +15V, –8V application, the positive VPOS = 15V, 25mA VNEG = –8V, 50mA channel requires a 4.7μF output capacitor and the negative
Figure 2. Switching Waveforms
channel requires at least 10μF of capacitance. 3487f 8