Datasheet LTC1773 (Analog Devices) - 9

LTC1773
U U W U APPLICATIONS INFORMATION
RMS capacitor current is given by: V – V 2 P IN OUT = I ( ) (1+ δ R) SYNC MAX ( ) V DS ON IN V [ (V –V )]12 OUT IN OUT C required I ≅ I where IN RMS MAX δ is the temperature dependency of RDS(ON) and K VIN is a constant inversely related to the gate drive current. This formula has a maximum at VIN = 2VOUT, where Both MOSFETs have I2R losses while the topside P-channel IRMS = IOUT/2. This simple worst-case condition is equation includes an additional term for transition losses, commonly used for design because even significant de- which are highest at high input voltages. The synchronous viations do not offer much relief. Note that capacitor MOSFET losses are greatest at high input voltage or during manufacturer’s ripple current ratings are often based on a short-circuit when the duty cycle in this switch is nearly 2000 hours of life. This makes it advisable to further derate 100%. the capacitor, or choose a capacitor rated at a higher The term (1 + δ) is generally given for a MOSFET in the temperature than required. Several capacitors may also be form of a normalized RDS(ON) vs temperature curve, but paralleled to meet size or height requirements in the δ = 0.005/°C can be used as an approximation for low design. Always consult the manufacturer if there is any voltage MOSFETs. CRSS is usually specified in the MOSFET question. characteristics. The constant K = 1.7 can be used to estimate the contributions of the two terms in the main
COUT Selection
switch dissipation equation. The selection of COUT is driven by the required effective Typical gate charge for the selected P-channel MOSFET series resistance (ESR). Typically, once the ESR require- should be less than 30nC (at 4.5VGS) while the turn-off ment is satisfied the capacitance is adequate for filtering. delay should be less than 150ns. However, due to differ- The output ripple (∆VOUT) is determined by: ences in test and specification methods of various MOSFET manufacturers, the P-channel MOSFET ultimately should ⎛ 1 ⎞ ∆VOUT ≅ ∆I ESR L + be evaluated in the actual LTC1773 application circuit to ⎝⎜ fC 8 OUT ⎠⎟ ensure proper operation. where f = operating frequency, C A Schottky diode can be placed in parallel with the syn- OUT = output capacitance and ∆I chronous MOSFET to improve efficiency. It conducts L = ripple current in the inductor. The output ripple is highest at maximum input voltage since ∆I during the dead-time between the conduction of the two L increases with input voltage. With ∆I power MOSFETs. This prevents the body diode of the L = 0.4IOUT(MAX) and allowing for 2/3 of the ripple due to ESR, the output ripple will be bottom MOSFET from turning on and storing charge less than 50mV at max V during the dead-time, which could cost as much as 1% in IN assuming: efficiency. A 1A Schottky is generally a good size for 5A to COUT required ESR < 2 RSENSE 8A regulators due to the relatively small average current. COUT > 1/(8fRSENSE) Larger diodes result in additional transition losses due to their larger junction capacitance. The diode may be omit- The first condition relates to the ripple current into the ESR ted if the efficiency loss can be tolerated. of the output capacitance while the second term guaran- tees that the output voltage does not significantly dis-
CIN Selection
charge during the operating frequency period due to ripple current. The choice of using smaller output capacitance In continuous mode, the source current of the top MOSFET increases the ripple voltage due to the discharging term is a square wave of duty cycle VOUT/VIN. To prevent large but can be compensated for by using capacitors of very voltage transients, a low ESR input capacitor sized for the low ESR to maintain the ripple voltage at or below 50mV. maximum RMS current must be used. The maximum 1773fb 9