FAN4603 Datasheet, PDF (10/12 Pages) Fairchild Semiconductor – 600mA, Fully Integrated, Buck Power Supply Module

Reducing PFM Output Ripple
PFM output ripple amplitude can be reduced by adding
external COUT, with negligible impact on efficiency.
Reduced output ripple also results in less DC voltage
excursion at very light loads. Maximum PFM ripple occurs at
no load and is VIN and VOUT proportional.
PCB Layout Guideline
Figure 28. Typical 1.82VOUT No Load PFM Ripple vs. VIN
Note:
4. Ripple is less for lower VOUT levels.
The effective value of external COUT for a desired ripple
amplitude can be determined using:
C_OUT. (μF)
=
⎜⎜⎝⎛
(V_IN
−
V_OUT ) • V_OUT
V_IN • V_R
•
62
⎟⎟⎠⎞
−1
(3)
where VR is the desired output ripple amplitude in mV and
VIN and VOUT are in Volts.
The simplified equation above is representative of nominal
component values and does not account for device
tolerances. The bias level effects associated with case size,
voltage rating, and dielectric type of ceramic capacitors
should be considered when selecting COUT.
Minimum Off-Time Effect on Switching
Frequency
tOFF(MIN) is 50ns, while tON(MIN) is 35nS This imposes
constraints on the maximum/minimum ^VOUT that the
V_IN
FAN4603 can provide while maintaining a fixed switching
frequency in PWM mode.
When VIN is LOW, fixed switching is maintained as long as
V_OUT
V_IN
≤ 1− t_{OFF (MIN )} • f_SW
≈ 0.7 .
When VIN is HIGH, fixed switching is maintained as long as
V _OUT
V _IN
≥ t_{ON (MIN )} • f _SW
≈ 0.2
The switching frequency drops when the regulator cannot
provide sufficient duty cycle at 6MHz to maintain regulation.
Lowering the switching frequency allows VOUT to remain in
regulation, even at very low or very high duty cycle.
Figure 29. Recommended PCB Layout
FB (pin 5) must be directly connected to VOUT (pin 4).
Figure 29 shows the recommended locations of optional CIN
and COUT, shown as 0603 size devices.
Pad P1, shown in Figure 30 “Land Pattern,” is an optional
GND pin. Connection is not required on the PCB.
© 2009 Fairchild Semiconductor Corporation
FAN4603 • Rev. 1.0.1
10
www.fairchildsemi.com