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BAT54 датащи(PDF) 9 Page - SamHop Microelectronics Corp. |
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BAT54 датащи(HTML) 9 Page - SamHop Microelectronics Corp. |
9 / 14 page SAMHOP Microelectronics Corp. 1.2MHz STEP-UP CONVERTER OPERATION The SM8237 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram in figure 1. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal pf the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered. Minimum output current The SM8237 can regulate three series LEDs connected at low output currents, down to approximately 4mA from a 4.2V supply, without pulse skipping, using the same external components as specified for 15mA operation. As current is further reduced, the device will begin skipping pulses. This will result in some low frequency ripple, although the LED current remains regulated on an average basis down to zero. APPLICATIONS INFORMATION Inductor selection A 22u H inductor is recommended for most SM8237 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire resistance). Some inductors in this category with small size are listed in Table 1. Capacitor selection The small size of ceramic capacitors makes them ideal for SM8237 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1μF input capacitor and a 0.22μF output capacitor are sufficient for most of SM8237 applicaiotns. Diode selection Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for SM8237 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while the diode capacitance (CT or CD ) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.2MHz switching frequency of the SM8237. A Schottky diode rated at 100mA to 200mA is sufficient for most SM8237 applications. Page 8 Part number LQH3C220 ELJPC220KF CDRH3D16-220 LB2012B220M LEM2520-220 DCR (Ω) 0.71 4.0 0.53 1.7 5.5 Current rating (mA) Panasonic 160 350 75 125 Manufacturer Murata 250 Sumida Taiyo Yuden Taiyo Yuden Part number CMDSH-3 CMDSH2-3 BAT54 Forward current (mA) 100 100 200 Voltage drop (V) 0.58 at 100mA 0.49 at 200mA 0.53 at 100mA Diode capacitance (pF) 7.0 at 10V 15 at 10V 10 at 25V Manufacturer Central Central Zetex Table 1. Recommended inductors Table 2. Recommended Schottky diodes V.1.3 Aug 28,2007 |
Аналогичный номер детали - BAT54 |
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Аналогичное описание - BAT54 |
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