Datasheet LT1912 (Analog Devices) - 9

LT1912
APPLICATIONS INFORMATION FB Resistor Network
switching frequency is because the LT1912 switch has The output voltage is programmed with a resistor divider finite minimum on and off times. The switch can turn on between the output and the FB pin. Choose the 1% resis- for a minimum of ~150ns and turn off for a minimum of tors according to: ~150ns. Typical minimum on time at 25°C is 80ns. This means that the minimum and maximum duty cycles are: ⎛ V R1 OUT ⎞ = R2 – 1 DCMIN = fSWtON(MIN) ⎝⎜ 0.79V ⎠⎟ DCMAX =1– fSWtOFF(MIN) Reference designators refer to the Block Diagram. where f
Setting the Switching Frequency
SW is the switching frequency, the tON(MIN) is the minimum switch on time (~150ns), and the tOFF(MIN) is The LT1912 uses a constant frequency PWM architecture the minimum switch off time (~150ns). These equations that can be programmed to switch from 200kHz to 500kHz show that duty cycle range increases when switching by using a resistor tied from the RT pin to ground. A table frequency is decreased. showing the necessary RT value for a desired switching A good choice of switching frequency should allow ad- frequency is in Figure 1. equate input voltage range (see next section) and keep
SWITCHING FREQUENCY (kHz) R
the inductor and capacitor values small.
T VALUE (kΩ)
200 187 300 121
Input Voltage Range
400 88.7 500 68.1 The maximum input voltage for LT1912 applications depends on switching frequency, the Absolute Maximum
Figure 1. Switching Frequency vs RT Value
Ratings of the VIN and BOOST pins, and the operating mode.
Operating Frequency Trade-Offs
While the output is in start-up, short-circuit, or other Selection of the operating frequency is a trade-off between overload conditions, the switching frequency should be efficiency, component size, minimum dropout voltage, and chosen according to the following equation. maximum input voltage. The advantage of high frequency VOUT + VD operation is that smaller inductor and capacitor values may VIN(MIN) = – V 1– f D + VSW be used. The disadvantages are lower efficiency, lower SW tOFF(MIN) maximum input voltage, and higher dropout voltage. The where V highest acceptable switching frequency (f IN(MAX) is the maximum operating input voltage, SW(MAX)) for a V given application can be calculated as follows: OUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max V load), f f D + VOUT SW is the switching frequency (set by RT), and SW(MAX) = t ( ) tON(MIN) is the minimum switch on time (~150ns). Note that ON(MIN) VD + VIN – VSW a higher switching frequency will depress the maximum where V operating input voltage. Conversely, a lower switching IN is the typical input voltage, VOUT is the output voltage, V frequency will be necessary to achieve safe operation at D is the catch diode drop (~0.5V) and VSW is the internal switch drop (~0.5V at max load). This equation high input voltages. shows that slower switching frequency is necessary to If the output is in regulation and no short-circuit, start- safely accommodate high VIN/VOUT ratio. Also, as shown up, or overload events are expected, then input voltage in the next section, lower frequency allows a lower dropout transients of up to 36V are acceptable regardless of the voltage. The reason input voltage range depends on the switching frequency. In this mode, the LT1912 may enter 1912fa 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts