Datasheet

 2012 Microchip Technology Inc. DS30684A-page 27
PIC18(L)F2X/45K50
2.4 Voltage Regulator Pins (VUSB3V3)
The on-chip voltage regulator must always be
connected directly to either a supply voltage or to an
external capacitor.
When the regulator is enabled (F devices), a low-ESR
(< 5Ω) capacitor is required on the V
USB3V3 pin to
stabilize the voltage regulator output voltage. The
V
USB3V3 pin must not be connected to VDD and is
recommended to use a ceramic capacitor of between
0.22 to 0.47 µF connected to ground.
It is recommended that the trace length not exceed
0.25 inch (6 mm). Refer to Section 29.0 “Electrical
Characteristics” for additional information.
When the regulator is disabled (LF devices), the
V
USB3V3 pin should be externally tied to a voltage
source maintained at the V
DD level. Refer to
Section 29.0 “Electrical Characteristics” for
information on VDD and VUSB3V3.
• LF devices (with the name, PIC18LF2X/45K50)
permanently disable the voltage regulator.
The V
DD level of these devices must comply with
the “voltage regulator disabled” specification for
Parameter D001, in Section 29.0 “Electrical
Characteristics”.
• F devices permanently enable the voltage
regulator.
These devices require an external capacitor on
the V
USB3V3 pin. It is recommended that the
capacitor be a ceramic cap between 0.22 to
0.47 µF.
2.4.1 CONSIDERATIONS FOR CERAMIC
CAPACITORS
In recent years, large value, low-voltage, surface-mount
ceramic capacitors have become very cost effective in
sizes up to a few tens of microfarad. The low-ESR, small
physical size and other properties make ceramic
capacitors very attractive in many types of applications.
Ceramic capacitors are suitable for use with the inter-
nal voltage regulator of this microcontroller. However,
some care is needed in selecting the capacitor to
ensure that it maintains sufficient capacitance over the
intended operating range of the application.
Typical low-cost, ceramic capacitors are available in
X5R, X7R and Y5V dielectric ratings (other types are
also available, but are less common). The initial toler-
ance specifications for these types of capacitors are
often specified as ±10% to ±20% (X5R and X7R), or
-20%/+80% (Y5V). However, the effective capacitance
that these capacitors provide in an application circuit will
also vary based on additional factors, such as the
applied DC bias voltage and the temperature. The total
in-circuit tolerance is, therefore, much wider than the
initial tolerance specification.
The X5R and X7R capacitors typically exhibit satisfac-
tory temperature stability (ex: ±15% over a wide
temperature range, but consult the manufacturer’s data
sheets for exact specifications). However, Y5V capaci-
tors typically have extreme temperature tolerance
specifications of +22%/-82%. Due to the extreme
temperature tolerance, a 10 F nominal rated Y5V type
capacitor may not deliver enough total capacitance to
meet minimum internal voltage regulator stability and
transient response requirements. Therefore, Y5V
capacitors are not recommended for use with the
internal regulator if the application must operate over a
wide temperature range.
In addition to temperature tolerance, the effective
capacitance of large value ceramic capacitors can vary
substantially, based on the amount of DC voltage
applied to the capacitor. This effect can be very signifi-
cant, but is often overlooked or is not always
documented.
A typical DC bias voltage vs. capacitance graph for
X7R type and Y5V type capacitors is shown in
Figure 2-3.
FIGURE 2-3: DC BIAS VOLTAGE vs.
CAPACITANCE
CHARACTERISTICS
When selecting a ceramic capacitor to be used with the
internal voltage regulator, it is suggested to select a
high-voltage rating, so that the operating voltage is a
small percentage of the maximum rated capacitor
voltage. For example, choose a ceramic capacitor
rated at 16V for the 3.3V V
USB3V3 voltage.
-80
-70
-60
-50
-40
-30
-20
-10
0
10
5 1011121314151617
DC Bias Voltage (VDC)
Capacitance Change (%)
01234 6789
16V Capacitor
10V Capacitor
6.3V Capacitor