TC1313 Datasheet, PDF (16/28 Pages) Microchip Technology – 500 mA Synchronous Buck Regulator, + 300 mA LDO

TC1313
5.0 APPLICATION
CIRCUITS/ISSUES
5.1 Typical Applications
The TC1313 500 mA buck regulator + 300 mA LDO
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 to 5V regulated
input applications. The 10-pin MSOP and 3X3 DFN
packages provide a small footprint with minimal exter-
nal components.
5.2 Fixed-Output Application
A typical V_OUT1 fixed-output voltage application is
shown in “Typical Application Circuits”. A 4.7 µF
V_IN1 ceramic input capacitor, 4.7 µF V_OUT1 ceramic
capacitor, 1.0 µF ceramic V_OUT2 capacitor and 4.7 µH
inductor make up the entire external component
solution for this dual-output application. No external
dividers or compensation components are necessary.
For this application, the input-voltage range is 2.7V to
4.2V, V_OUT1 = 1.5V at 500 mA, while V_OUT2 = 2.5V at
300 mA.
5.3 Adjustable-Output Application
A typical V_OUT1 adjustable-output application is also
shown in “Typical Application Circuits”. For this
application, the buck regulator output voltage is adjust-
able by using two external resistors as a voltage
divider. For adjustable-output voltages, it is recom-
mended that the top resistor divider value be 200 kΩ.
The bottom resistor divider can be calculated using the
following formula:
EQUATION 5-1:
R_BOT
=
R_TOP
×
⎛
⎝
V^----_O^----_U^----_T^V---₁^{-F---–B----}V^----_F^---_B^{--⎠⎞}
Example:
R_TOP = 200 kΩ
V_OUT1 = 2.1V
V_FB = 0.8V
R_BOT = 200 kΩ x (0.8V/(2.1V – 0.8V))
R_BOT = 123 kΩ (Standard Value = 121 kΩ)
For adjustable output applications, an additional R-C
compensation is necessary for the buck regulator
control loop stability. Recommended values are:
R_COMP = 4.99 kΩ
C_COMP = 33 pF
An additional V_IN2 capacitor can be added to reduce
high-frequency noise on the LDO input-voltage pin
(V_IN2). This additional capacitor (1 µF) is not necessary
for typical applications.
5.4 Input and Output Capacitor
Selection
As with all buck-derived dc-dc switching regulators, the
input current is pulled from the source in pulses. This
places a burden on the TC1313 input filter capacitor. In
most applications, a minimum of 4.7 µF is recom-
mended on V_IN1 (buck regulator input-voltage pin). In
applications that have high source impedance, or have
long leads (10 inches) connecting to the input source,
additional capacitance should be used. The capacitor
type can be electrolytic (aluminum, tantalum, POSCAP,
OSCON) or ceramic. For most portable electronic
applications, ceramic capacitors are preferred due to
their small size and low cost.
For applications that require very low noise on the LDO
output, an additional capacitor (typically 1 µF) can be
added to the V_IN2 pin (LDO input voltage pin).
Low ESR electrolytic or ceramic can be used for the
buck regulator output capacitor. Again, ceramic is
recommended because of its physical attributes and
cost. For most applications, a 4.7 µF is recommended.
Refer to Table 5-1 for recommended values. Larger
capacitors (up to 22 µF) can be used. There are some
advantages in load step performance when using
larger value capacitors. Ceramic materials, X7R and
X5R, have low temperature coefficients and are well
within the acceptable ESR range required.
TABLE 5-1: TC1313 RECOMMENDED
CAPACITOR VALUES
C (V_IN1) C (V_IN2) C_OUT1 C_OUT2
Min 4.7 µF none 4.7 µF 1 µF
Max
none none 22 µF 10 µF
DS21974A-page 16
© 2005 Microchip Technology Inc.