Datasheet LT3475, LT3475-1 (Analog Devices) - 9

LT3475/LT3475-1
APPLICATIONS INFORMATION
Table 1 shows suggested RT selections for a variety of The maximum operating voltage is determined by the switching frequencies. absolute maximum ratings of the VIN and BOOST pins, and by the minimum duty cycle.
Operating Frequency Selection
V V OUT + VF – V The choice of operating frequency is determined by IN MAX ( ) = DC F + VSW MIN several factors. There is a tradeoff between effi ciency and component size. A higher switching frequency allows the with DCMIN = tON(MIN) • f use of smaller inductors at the cost of increased switching where t losses and decreased effi ciency. ON(MIN) is equal to 140ns and f is the switching frequency. Another consideration is the maximum duty cycle. In certain Example: f = 750kHz, V applications, the converter needs to operate at a high duty OUT = 3.4V cycle in order to work at the lowest input voltage possible. DCMIN = 140ns • 750kHz = 0.105 The LT3475 has a fi xed oscillator off time and a variable 3.4V + 0.4V on time. As a result, the maximum duty cycle increases V – 0.4V IN MAX ( ) = + 0.4V = 36V 0.105 as the switching frequency is decreased. The minimum duty cycle depends on the switching fre-
Input Voltage Range
quency. Running at a lower switching frequency might The minimum operating voltage is determined either by the allow a higher maximum operating voltage. Note that LT3475’s undervoltage lockout of 4V, or by its maximum this is a restriction on the operating input voltage; the duty cycle. The duty cycle is the fraction of time that the circuit will tolerate transient inputs up to the Absolute internal switch is on and is determined by the input and Maximum Ratings of the VIN and BOOST pins. The input output voltages: voltage should be limited to the VIN operating range (36V) V ( ) during overload conditions (short circuit or start up). DC OUT + VF = V ( ) IN – VSW + VF
Minimum On Time
where V The LT3475 will regulate the output current at input volt- F is the forward voltage drop of the catch diode (~0.4V) and V ages greater than V SW is the voltage drop of the internal switch IN(MAX). For example, an application (~0.4V at maximum load). This leads to a minimum input with an output voltage of 3V and switching frequency of voltage of: 1.2MHz has a VIN(MAX) of 20V, as shown in Figure 3. Figure 4 shows operation at 35V. Output ripple and peak inductor V V OUT + VF – V IN MIN ( ) = DC F + VSW MAX with DCMAX = 1–tOFF(MIN) • f VOUT 500mV/DIV where t (AC COUPLED) 0FF(MIN) is equal to 167ns and f is the switching frequency. IL 1A/DIV Example: f = 600kHz, VOUT = 4V DC VSW MAX = 1− 167ns • 600kHz = 0.90 20V/DIV 4V + 0.4V V – 0.4V 3475 F03 IN MIN ( ) = + 0.4V = 4.9V 0.9
Figure 3. Operation at VIN(MAX) = 20V. VOUT = 3V and fSW = 1.2MHHz
3475fb 9