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LTR3800 Datasheet(PDF) 11 Page - Linear Technology |
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LTR3800 Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page 11 LTC1778/LTC1778-1 1778fb APPLICATIO S I FOR ATIO with temperature, typically about 0.4%/°C as shown in Figure 2. For a maximum junction temperature of 100°C, using a value ρT = 1.3 is reasonable. The power dissipated by the top and bottom MOSFETs strongly depends upon their respective duty cycles and the load current. When the LTC1778 is operating in continuous mode, the duty cycles for the MOSFETs are: D V V D VV V TOP OUT IN BOT IN OUT IN = = – The resulting power dissipation in the MOSFETs at maxi- mum output current are: PTOP = DTOP IOUT(MAX)2 ρT(TOP) RDS(ON)(MAX) + k VIN2 IOUT(MAX) CRSS f PBOT = DBOT IOUT(MAX)2 ρT(BOT) RDS(ON)(MAX) Both MOSFETs have I2R losses and the top MOSFET includes an additional term for transition losses, which are largest at high input voltages. The constant k = 1.7A–1 can be used to estimate the amount of transition loss. The bottom MOSFET losses are greatest when the bottom duty cycle is near 100%, during a short-circuit or at high input voltage. Operating Frequency The choice of operating frequency is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses but requires larger inductance and/or capacitance in order to maintain low output ripple voltage. The operating frequency of LTC1778 applications is deter- mined implicitly by the one-shot timer that controls the on-time tON of the top MOSFET switch. The on-time is set by the current into the ION pin and the voltage at the VON pin (LTC1778-1) according to: t V I pF ON VON ION = () 10 VON defaults to 0.7V in the LTC1778. Tying a resistor RON from VIN to the ION pin yields an on- time inversely proportional to VIN. For a step-down con- verter, this results in approximately constant frequency operation as the input supply varies: f V VR pF H OUT VON ON Z = () [] 10 To hold frequency constant during output voltage changes, tie the VON pin to VOUT or to a resistive divider from VOUT when VOUT > 2.4V. The VON pin has internal clamps that limit its input to the one-shot timer. If the pin is tied below 0.7V, the input to the one-shot is clamped at 0.7V. Simi- larly, if the pin is tied above 2.4V, the input is clamped at 2.4V. In high VOUT applications, tying VON to INTVCC so that the comparator input is 2.4V results in a lower value for RON. Figures 3a and 3b show how RON relates to switching frequency for several common output voltages. RON (kΩ) 100 100 1000 1000 10000 1778 F03a VOUT = 3.3V VOUT = 1.5V VOUT = 2.5V RON (kΩ) 100 100 1000 1000 10000 1778 F03b VOUT = 3.3V VOUT = 12V VOUT = 5V Figure 3a. Switching Frequency vs RON for the LTC1778 and LTC1778-1 (VON = 0V) Figure 3b. Switching Frequency vs RON for the LTC1778-1 (VON = INTVCC) |
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