Datasheet LT3474, LT3474-1 (Analog Devices) - 10

LT3474/LT3474-1
APPLICATIONS INFORMATION Setting the Switching Frequency
V ( + V ) DC OUT F = The LT3474 uses a constant frequency architecture that V – V ( + V IN SW F ) can be programmed over a 200kHz to 2MHz range with a single external timing resistor from the RT pin to ground. The current that fl ows into the timing resistor is used where VF is the forward voltage drop of the catch diode to charge an internal oscillator capacitor. A graph for (~0.4V) and VSW is the voltage drop of the internal switch selecting the value of RT for a given operating frequency (~0.4V at maximum load). This leads to a minimum input is shown in the Typical Performance Characteristics voltage of: section. Table 1 shows suggested RT selections for a V + V variety of switching frequencies. V OUT F – V + V IN MIN DC F SW ( ) = MAX
Table 1. Switching Frequencies
with DCMAX = 1–tOFF(MIN) • f
SWITCHING FREQUENCY (MHz) RT (kΩ)
2 10 where t0FF(MIN) is equal to 200ns and f is the switching 1.5 18.7 frequency. 1 33.2 Example: f = 500kHz, VOUT = 4V 0.7 52.3 DC = 1− ns 200 • k 500 Hz = 0 9 . 0 0.5 80.6 MAX 0.3 147 4V + 0 4 . V V – 0 4 . V + 0 4 . V = 4 9 . V IN MIN = 0.2 232 ( ) 0.9 The maximum operating voltage is determined by the
Operating Frequency Selection
absolute maximum ratings of the VIN and BOOST pins, The choice of operating frequency is determined by sev- and by the minimum duty cycle. eral factors. There is a tradeoff between effi ciency and V + V component size. Higher switching frequency allows the V OUT F – V + V IN MAX F SW ( ) = use of smaller inductors at the cost of increased switching DC MIN losses and decreased effi ciency. with DCMIN = tON(MIN) • f Another consideration is the maximum duty cycle. In where t certain applications, the converter needs to operate at a ON(MIN) is equal to 160ns and f is the switching frequency. high duty cycle in order to work at the lowest input voltage possible. The LT3474 has a fi xed oscillator off-time and Example: f = 500kHz, VOUT = 2.5V a variable on-time. As a result, the maximum duty cycle DC = ns 160 • k 500 Hz = 0 0 . 8 increases as the switching frequency is decreased. MIN 2 5 . V + 0 4 . V V = – 0 4 . V + 0 4 . V = 36V
Input Voltage Range
IN MAX ( ) 0.0 08 The minimum operating voltage is determined either by the The minimum duty cycle depends on the switching fre- LT3474’s undervoltage lockout of 4V, or by its maximum quency. Running at a lower switching frequency might duty cycle. The duty cycle is the fraction of time that the allow a higher maximum operating voltage. Note that this internal switch is on and is determined by the input and is a restriction on the operating input voltage; the circuit output voltages: will tolerate transient inputs up to the Absolute Maximum Rating. 3474fd 10