ltc3521fe Linear Technology Corporation, ltc3521fe Datasheet - Page 11

• Manufacturer Part Number: ltc3521fe
• Description: Ltc3521 Wide Vin, 1a Buck-boost Dc/dc And Dual 600ma Buck Dc/dc Converters Features
• Manufacturer: Linear Technology Corporation

operaTion
modes. These advantages result in increased efficiency
and stability in comparison to the traditional 4-switch
buck-boost converter.
Error Amplifier and Compensation
The buck-boost converter utilizes a voltage mode error
amplifier with an internal compensation network as shown
in Figure 2.
Notice that resistor R2 of the external resistor divider
network plays an integral role in determining the frequency
response of the compensation network. The ratio of R2 to
R1 must be set to program the desired output voltage but
this still allows the value of R2 to be adjusted to optimize
the transient response of the converter. Increasing the value
of R2 generally leads to greater stability at the expense of
reduced transient response speed. Increasing the value of
R2 can yield substantial transient response improvement in
cases where the phase margin has been reduced due to the
use of a small value output capacitor or a large inductance
(particularly with large boost step-up ratios). Conversely,
decreasing the value of R2 increases the loop bandwidth
which can improve the speed of the converter’s transient
response. This can be useful in improving the transient
response if a large valued output capacitor is utilized. In
Figure 2. Buck-Boost Error Amplifier and Compensation
LTC3521
+
–
GND
0.6V
PV
OUT
FB1
3521 F02
R2
R1
V
OUT
this case, the increased bandwidth created by decreasing
R2 is used to counteract the reduced converter bandwidth
caused by the large output capacitor.
Current Limit Operation
The buck-boost converter has two current limit circuits.
The primary current limit is an average current limit circuit
which injects an amount of current into the feedback node
which is proportional to the extent that the switch A cur-
rent exceeds the current limit value. Due to the high gain
of this loop, the injected current forces the error amplifier
output to decrease until the average current through switch
A decreases approximately to the current limit value. The
average current limit utilizes the error amplifier in an ac-
tive state and thereby provides a smooth recovery with
little overshoot once the current limit fault condition is
removed. Since the current limit is based on the average
current through switch A, the peak inductor current in
current limit will have a dependency on the duty cycle
(i.e., on the input and output voltages in the overcurrent
condition).
The speed of the average current limit circuit is limited by
the dynamics of the error amplifier. On a hard output short,
it would be possible for the inductor current to increase
substantially beyond current limit before the average cur-
rent limit circuit would react. For this reason, there is a
second current limit circuit which turns off switch A if the
current ever exceeds approximately 165% of the average
current limit value. This provides additional protection in
the case of an instantaneous hard output short.
Reverse Current Limit
The reverse current comparator on switch D monitors
the inductor current entering PV
exceeds 375mA (typical), switch D will be turned off for
the remainder of the switching cycle.
OUT
. When this current
LTC3521

3521f