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MCP1603
DS22042A-page 14
2007 Microchip Technology Inc.
5.0
APPLICATION INFORMATION
5.1
Typical Applications
The MCP1603 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 2x3
DFN packages provide a small footprint with minimal
external components.
5.2
Fixed Output Voltage Applications
in an application used to convert three NiMH batteries
into a well regulated 1.8V @ 500 mA output. A 4.7 F
input capacitor, 4.7 F output capacitor, and a 4.7 H
inductor make up the entire external component solu-
tion 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 can be used. The circuit listed in
Figure 6-2shows an adjustable MCP1603 being used to convert a
5V rail to 1.0V @ 500 mA. The output voltage is adjust-
able by using two external resistors as a voltage
divider.
For
adjustable-output
voltages,
it
is
recommended that the top resistor divider value be
200 k
Ω. 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 RCOMP and CCOMP.
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 (VOUT) to
input voltage (VIN) relationship of VOUT/VIN. To prevent
undesirable input voltage transients, the input capacitor
should be a low ESR type with an RMS current rating
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
EQUATION 5-3:
RBOT
RTOP
VFB
VOUT VFB
–
-----------------------------
×
=
Example:
RTOP
=200 k
Ω
VOUT
=1.0V
VFB
=0.8V
RBOT
=200 k
Ω x (0.8V/(1.0V - 0.8V))
RBOT
=800 k
Ω
(Standard Value = 787 k
Ω)
RCOMP
=4.99 k
Ω
CCOMP
=33 pF
ICIN RMS
,
IOUT MAX
,
VOUT
VIN VOUT
–
()
×
VIN
------------------------------------------------------
×
=
ΔV
OUT
ΔI
L
ESR
×
ΔI
L
8fC
×
---------------------
+
=