Datasheet LTC1625 (Analog Devices) - 10

LTC1625
U U W U APPLICATIONS INFORMATION
V 7V Top Duty Cycle OUT = VIN V V Bottom Duty Cycle IN OUT = – 1.2V VIN The MOSFET power dissipations at maximum output 1µs 4µs current are: ± 1625 F04 0  V  2
Figure 4. SYNC Clock Waveform
P OUT = I ( )(ρ ) R TOP O MAX ( ) T(TOP ( ) DS ON )  ( ) VIN 
Inductor Value Selection
+ 2 k ( )(V ) I ( ) C ( )(f IN O MAX ( ) RSS ) Given the desired input and output voltages, the inductor value and operating frequency directly determine the  ripple current: V – V  2 P IN OUT = I ( )(ρ ) R BOT O MAX ( ) T BOT ( ) DS ON )  ( ) VIN      ∆ V V I OUT OUT L = 1–  f L   VIN  ( )( ) Both MOSFETs have I2R losses and the PTOP equation includes an additional term for transition losses, which are Lower ripple current reduces core losses in the inductor, largest at high input voltages. The constant k = 1.7 can be ESR losses in the output capacitors and output voltage used to estimate the amount of transition loss. The bottom ripple. Thus, highest efficiency operation is obtained at MOSFET losses are greatest at high input voltage or during low frequency with small ripple current. To achieve this, a short circuit when the duty cycle is nearly 100%. however, requires a large inductor. A reasonable starting point is to choose a ripple current
Operating Frequency and Synchronization
that is about 40% of IO(MAX). Note that the largest ripple The choice of operating frequency and inductor value is a current occurs at the highest VIN. To guarantee that ripple trade-off between efficiency and component size. Low current does not exceed a specified maximum, the induc- frequency operation improves efficiency by reducing tor should be chosen according to: MOSFET switching losses, both gate charge loss and transition loss. However, lower frequency operation  V   V  requires more inductance for a given amount of ripple L OUT OUT ≥   1–  ∆ current. (f)( I ) V  L MAX ( )   IN MAX ( ) The internal oscillator runs at a nominal 150kHz frequency
Burst Mode Operation Considerations
when the SYNC pin is left open or connected to ground. Pulling the SYNC pin above 1.2V will increase the fre- The choice of RDS(ON) and inductor value also determines quency by 50%. The oscillator will injection lock to a clock the load current at which the LTC1625 enters Burst Mode signal applied to the SYNC pin with a frequency between operation. When bursting, the controller clamps the peak 165kHz and 200kHz. The clock high level must exceed inductor current to approximately: 1.2V for at least 1µs and no longer than 4µs as shown in Figure 4. The top MOSFET turn-on will synchronize with mV I = 30 the rising edge of the clock. BURST PEAK ( ) RDS ON ( ) 10