LTC3115-2_15 Datasheet, PDF (29/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Applications Information
The final step in the design process is to compute the Bode
plot for the entire loop using the designed compensation
network and confirm its phase margin and crossover
frequency. The complete loop Bode plot for this example
is shown in Figure 14. The loop crossover frequency is
22kHz which is close to the design target and the phase
margin is approximately 60°.
The Bode plot for the complete loop should be checked over
all operating conditions and for variations in component
values to ensure that sufficient phase margin exists in all
cases. The stability of the loop should also be confirmed
via time domain simulation and by evaluating the transient
response of the converter in the actual circuit.
Output Voltage Programming
The output voltage is set via the external resistor divider
comprised of resistors R_TOP and R_BOT as show in Figures 8
and 9. The resistor divider values determine the output
regulation voltage according to:
V_OUT
=
1.000V
⎛
⎝⎜
1+
R_TOP
R_BOT
⎞
⎠⎟
60
180
PHASE
40
120
20
60
GAIN
0
0
–20
–60
–40
–60
10
–120
f_C
100
1k
10k 100k
FREQUENCY (Hz)
–180
1M
31152 F14
Figure 14. Complete Loop Bode Plot
In addition to setting the output voltage, the value of
R_TOP is instrumental in controlling the dynamics of the
compensation network. When changing the value of this
resistor, care must be taken to understand the impact this
will have on the compensation network.
In addition to setting the output voltage, the Thevenin
equivalent resistance of the resistor divider determines the
gain of the current limit. For applications with a top divider
resistor (R_TOP) of less than 1M, it is recommended that
components R1 and C1 be added at the FB pin as shown in
Figure 15 to maintain the gain of the current limit loop and
minimize overshot on recovery from output short circuits.
This additional circuit has no impact on compensation of
the control loop.
V_OUT
LTC3115-2
VC
FB
31152 F15
R_FB ^CFB
R1
100k
C1
1000pF
R_TOP R_FF
C_FF
R_BOT
Figure 15. R1 and C1 are Recommended if R_TOP < 1MΩ
Switching Frequency Selection
The switching frequency is set by the value of a resistor
connected between the RT pin and ground. The switching
frequency, f, is related to the resistor value by the following
equation where R_T is the resistance:
f
=
35.7MHz
(R_T/ kΩ )
Higher switching frequencies facilitate the use of smaller
inductors as well as smaller input and output filter capaci-
tors which results in a smaller solution size and reduced
component height. However, higher switching frequencies
also generally reduce conversion efficiency due to the
increased switching losses.
In addition, higher switching frequencies (above 750kHz)
will reduce the maximum output current that can be sup-
plied (see Typical Performance Characteristics for details).
For applications with V_OUT ≥ 20V, a maximum switching
frequency of 1MHz is recommended.
For more information www.linear.com/LTC3115-2
31152fa
29