LTC3118_15 Datasheet, PDF (29/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Applications Information
the LTC3118 buck-boost converter will generate an aver-
age 3.6A of inductor current (typical) from the converter
making the transconductance gain 6A /V. As with peak
current mode control, the inner average current control
loop effectively turns the inductor into a current source over
the frequency range of interest, resulting in a frequency
response from the power stage that exhibits a single pole
(–20dB / decade) roll-off. The output capacitor (C_OUT) and
load resistance (R_LOAD) form a dominant low frequency
pole, where the effective series resistance of the output
capacitor and its capacitance form a zero, usually at a high
enough frequency to be ignored.
A potentially troublesome right half plane zero (RHPZ)
is also encountered if the converter is operated in boost
mode. The RHPZ causes an increase in gain, like a zero, but
a decrease in phase, like a pole. This can ultimately limit
the maximum converter bandwidth that can be achieved
with the LTC3118. The RHPZ is not present when operat-
ing in buck mode.
The overall open loop gain at DC is the product of the
following terms:
Voltage Error Amp Gain:
g_m
• R_EA
=
80µS
•
5MΩ
=
400V
V
(fixed)
Voltage Divider Gain:
V_FB
V_OUT
=
1V
V_OUT
Current Loop Transconductance:
g_c
=
6A
V
(fixed)
Load Resistance:
R_LOAD
=
V_OUT
I_LOAD
The frequency dependent terms that affect the loop gain
include:
Output Load Pole (P1):
1
(application dependent)
2π • R_LOAD • C_OUT
Right Half Plane Zero (RHPZ):
V_IN ² • R_L
V_OUT ² • 2π • L
(application dependent)
Voltage Error Amplifier Compensation: 2 poles and 1 zero
(application dependent)
The voltage amplifier’s frequency response is designed
to optimize the response for the overall loop. Measure-
ment of the power stage gain over line, load, component
variation and frequency is strongly recommended prior
to loop design. The design parameters for compensation
design will focus on the series resistor and capacitors
connected from VC to ground (R_Z, C_P1 and C_P2). Being a
buck-boost converter, the target loop crossover frequency
for the compensation design will be dictated by the high-
est boost ratio and load current that is expected, as this
will result in the lowest RHPZ frequency. The general goal
is to set the crossover frequency and provide sufficient
phase boost using the external compensation network.
For more information www.linear.com/LTC3118
3118f
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