LTC3118_15 Datasheet, PDF (28/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Applications Information
Ceramic capacitors are often utilized in switching con-
verter applications due to their small size, low ESR and
low leakage currents. However, many ceramic capacitors
intended for power applications experience a significant
loss in capacitance from their rated value as the DC bias
voltage on the capacitor increases. It is not uncommon
for a small surface mount capacitor to lose more than
50% of its rated capacitance when operated near its
maximum rated voltage. This effect is generally reduced
as the case size is increased for the same nominal value
capacitor. As a result, it is often necessary to use a larger
value capacitance or a higher voltage rated capacitor than
would ordinarily be required to actually realize the intended
capacitance at the operating voltage of the application. X5R
and X7R dielectric types are recommended as they exhibit
the best performance over the wide operating range and
temperature of the LTC3118. To verify that the intended
capacitance is achieved in the application circuit, be sure
to consult the capacitor vendor’s curve of capacitance
versus DC bias voltage.
Compensation of the Buck-Boost Converter
The LTC3118 utilizes an average current architecture to
regulate the output voltage. Average current mode control
has two loops that require frequency compensation, the
inner average current loop and the outer voltage loop. The
compensation for the inner average current loop is fixed
within the LTC3118 to simplify the loop design and provide
the highest possible bandwidth over a wide operating range.
The outer voltage loop does require external compensation
components, allowing the overall loop characteristics to
be customized for the application.
The average current mode control used in the LTC3118 can
be conceptualized as a voltage-controlled current source
(V_CCS), driving the output load formed primarily by R_LOAD
and C_OUT, as shown in Figure 8.
The voltage error amplifier output (VC), provides a com-
mand input to the V_CCS. The full-scale range of VC is 0.6V
(200mV to 800mV). With a full-scale command on VC,
VOLTAGE
CONTROLLED
CURRENT
SOURCE
+
–
VOLTAGE
ERROR
AMP
g_m
g_m = 3.6A/0.6V
800mV
FB
1V
VC
GND
R1
400k
R2
100k
C_OUT
47µF
R_COESR
0.01Ω
V_OUT = 5V
R_LOAD
5Ω
R_Z
C_P1
C_P2
3118 F08
Figure 8. Simplified Representation of Average Current Mode Control Loop
3118f
28
For more information www.linear.com/LTC3118