Page 2 of 9
antenna, picking up excessive noise. For improved results, the test hook should be
removed from the tip of the probe. The tip should be touched against the output turret,
with the bare ground shield pressed against the ground turret. This reduces the noise seen
on the waveform.
Note 1: The MSK5059RH has a typical minimum on time requirement of 300nS
corresponding to a minimum duty cycle of 15% at 500kHz switching frequency. Forcing
the device to operate at less than the minimum on time may result in irregular switching
waveforms and present the appearance of instability. The default configuration for this
evaluation card is 1.8V out and it may present irregular switching waveforms at input
voltages greater than 12V. When configured for an output voltage of 2.5V or greater the
MSK5059RH will function normally with input voltages up to the maximum rating of
15V. If operating the MSK5059RH at less than the minimum on time is required greater
than typical compensation can reduce the irregular switching.
Output Voltage Programming
V
OUT
= V
FB
* (1+R1/R2)
R1 = R2 * (V
OUT
/V
FB
-1)
Given: V
REF
= 1.21V Typ.
Factory Configuration: R1 = 1.21K, R2 = 2.49K
V
OUT
= 1.21 * (1+1.21/2.49) = 1.8V
Efficiency
Typical efficiency curves for 1.8V and 3.3V output voltages with 5V
IN
are shown in
Figure 1.
Figure 1
Boost Pin
The Boost pin provides drive voltage greater than V
IN
to the base of the power transistor.
Using a voltage greater than V
IN
ensures hard saturation of the power switch significantly
improving overall efficiency. Connect a capacitor between Boost and SW to store a
charge. Connect a diode between V
IN
and Boost to charge the capacitor during the off
V
in
= 5V
L = 6.8µH