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Figures 4-A and B illustrate typical results for a step load function between 500ma and
2.0A. Notice that the ringing in Figure 4-B is evident for the design intentionally
modified to be less stable versus the critically damped response in Figure 4-A.
Figure 4-A (Typical Response)
Figure 4-B (Modified Response)
Current Sharing and Synchronization
There are several advantages to using a multiple switcher approach compared to a single
larger switcher. The inductor size is considerably reduced. Three 4A inductors store less
energy (LI
2
/2) than one 12A coil so are far smaller. In addition, synchronizing three
converters 120° out of phase with each other reduces input and output ripple currents.
This reduces the ripple rating, size and cost of filter capacitors. If the SYNC pin is not
used in the application, tie it to ground. To synchronize switching to an external clock,
apply a logic-level signal to the SYNC pin. The amplitude must be from a logic low to
greater than 2.2V, with a duty cycle between 10% and 90%. The synchronization
frequency must be greater than the free-running oscillator frequency and less than
1 MHz. This means that minimum practical sync frequency is equal to the worst-case
high self-oscillating frequency (560 kHz), not the typical operating frequency of
500 kHz. Caution should be used when synchronizing above 700 kHz because at higher
sync frequencies the amplitude of the internal slope compensation used to prevent
subharmonic switching is reduced. Additional circuitry may be required to prevent
subharmonic oscillation
Shutdown
For normal operation, the SHDN pin can be left floating. SHDN has two output-disable
modes: lockout and shutdown. When the pin is taken below the lockout threshold,
switching is disabled. This is typically used for input undervoltage lockout. Grounding
the SHDN pin places the RH1959 in shutdown mode. This reduces total board supply
current to 20µA.
Input/Output Capacitors
The input capacitors C7A and B are 47µF tantalum capacitors and were chosen due to
their low ESR, and effective low frequency filtering. The input ripple current for a buck