Datasheet

 2007-2012 Microchip Technology Inc. DS22042B-page 17
MCP1603/B/L
5.0 APPLICATION INFORMATION
5.1 Typical Applications
The MCP1603/B/L 500 mA synchronous buck
regulator operates over a wide input voltage range
(2.7V to 5.5V) and is ideal for single-cell Li-Ion battery-
powered applications, USB-powered applications,
three cell NiMH or NiCd applications and 3V or 5V
regulated input applications. The 5-lead TSOT and 8-
lead 2 x 3 DFN packages provide a small footprint with
minimal external components.
5.2 Fixed Output Voltage Applications
The Typical Application Circuit shows a fixed
MCP1603/B/L in an application used to convert three
NiMH batteries into a well-regulated 1.8V @ 500 mA
output. A 4.7 µF input capacitor, 4.7 µF output
capacitor, and a 4.7 µH inductor make up the entire
external component solution for this application. No
external voltage divider or compensation is necessary.
In addition to the fixed 1.8V option, the MCP1603 is
also available in 1.2V, 1.5V, 2.5V, or 3.3V fixed voltage
options.
5.3 Adjustable Output Voltage
Applications
When the desired output for a particular application is
not covered by the fixed-voltage options, an adjustable
MCP1603/B/L can be used. The circuit listed in
Figure 6-2 shows an adjustable device being used to
convert a 5V rail to 1.0V @ 500 mA. The output voltage
is adjustable by using two external resistors as a volt-
age divider. For adjustable-output voltages, it is
recommended that the top resistor divider value be
200 k. The bottom resistor value can be calculated
using the following equation:
EQUATION 5-1:
For adjustable output applications, an additional R-C
compensation network is necessary for control loop
stability. Recommended values for any output voltage
are:
Refer to Figure 6-2 for proper placement of R
COMP
and
C
COMP
.
5.4 Input Capacitor Selection
The input current to a buck converter, when operating
in Continuous Conduction mode, is a squarewave with
a duty cycle defined by the output voltage (V
OUT
) to
input voltage (V
IN
) relationship of V
OUT
/V
IN
. To prevent
undesirable input voltage transients, the input capacitor
should be a low-ESR type with an RMS current rating
given by Equation 5.5. Because of their small size and
low ESR, ceramic capacitors are often used. Ceramic
material X5R or X7R are well suited, since they have a
low-temperature coefficient and acceptable ESR.
EQUATION 5-2:
Table 5-1 contains the recommend range for the input
capacitor value.
5.5 Output Capacitor Selection
The output capacitor helps provide a stable output
voltage during sudden load transients, smooths the
current that flows from the inductor to the load, and
reduces the output voltage ripple. Therefore, low-ESR
capacitors are a desirable choice for the output capac-
itor. As with the input capacitor, X5R and X7R ceramic
capacitors are well suited for this application.
The output ripple voltage is often a design specifica-
tion. A buck converters’ output ripple voltage is a
function of the charging and discharging of the output
capacitor and the ESR of the capacitor. This ripple
voltage can be calculated by Equation 5-3.
EQUATION 5-3:
Table 5-1 contains the recommend range for the output
capacitor value.
R
BOT
R
TOP
V
FB
V
OUT
V
FB
–
---------------------------- -


=
Example:
R
TOP
=200k
V
OUT
=1.0V
V
FB
=0.8V
R
BOT
=200k x (0.8V/(1.0V – 0.8V))
R
BOT
=800k(Standard Value = 787 k)
R
COMP
=4.99k
C
COMP
=33pF
TABLE 5-1: CAPACITOR VALUE RANGE
C
IN
C
OUT
Minimum 4.7 µF 4.7 µF
Maximum — 22 µF
I
CIN RMS,
I
OUT MAX,
V
OUT
V
IN
V
OUT
–
V
IN
------------------------------------------------------



=
 V
OUT
 I
L
ESR 
 I
L
8fC

-------------------- -+=