LTC3536_15 Datasheet, PDF (10/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Operation
diode from the RT pin to V_IN can be used in addition to
current limiting resistor. For noise sensitive applications,
an internal phase-locked loop allows the LTC3536 to
be synchronized to an external clock signal applied to
the MODE/SYNC pin. The free-running frequency of the
oscillator can be programmed slower or faster than the
synchronization clock frequency.
Whether operating from its internal oscillator or when
synchronized to an external clock signal, the LTC3536 is
able to operate with a switching frequency from 300kHz
to 2MHz, providing the ability to minimize the size of the
external components and optimize the power conversion
efficiency.
Error Amplifier
The LTC3536 has an internal high gain operational ampli-
fier which provides frequency compensation of the control
loop that maintains output voltage regulation. To ensure
stability of this control loop, an external compensation
network must be installed in the application circuit. A
Type III compensation network as shown in Figure 2 is
recommended for most applications since it provides the
flexibility to optimize the converter’s transient response
while simultaneously minimizing any DC error in the output
voltage. Details on designing the compensation network
in LTC3536 applications can be found in the Applications
Information section of this data sheet.
V_OUT
R_TOP
R_BOT
R_FF
C_FF
C_{FB RFB}
C_POLE
0.6V
FB
VC
LTC3536
+
PWM
–
GND
3536 F02
Figure 2. Error Amplifier and Compensation Network
Input and Peak Current Limits
The LTC3536 has two current limit circuits that are de-
signed to limit the peak inductor current to ensure that
the switch currents remain within the capabilities of the
IC during output short-circuit or overload conditions. The
10
input current limit operates by injecting a current into the
feedback pin, which is proportional to the extent that the
inductor current exceeds the input current limit threshold
(typically 2.5A). Due to the high gain of the feedback loop,
this injected current forces the error amplifier output to
decrease until the average current through the inductor
is approximately reduced to the current limit threshold.
For this current limit feature to be most effective, the
Thevenin resistance (R_BOT//R_TOP) from FB to ground
should exceed 100kΩ.
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
CURRENT OUT OF SW1 PIN (A)
3536 F03
Figure 3. FB Current for Input Current Limitation
Since this input current limit circuit maintains the error
amplifier in an active state it ensures a smooth recovery
and minimal overshoot once the current limit fault condi-
tion is removed. On a hard output short, it is possible for
the inductor current to increase substantially beyond the
current limit threshold before the input current limit has
time to react and reduce the inductor current. For this
reason, there is a second current limit circuit (peak cur-
rent limit), which turns off power switch A if the current
through switch A exceeds the approximately 3.4A limit
threshold. This provides additional protection in the case of
an instantaneous hard output short and provides time for
the primary current limit to react. When the input voltage
is lower than 2.4V, the input and peak current limit thresh-
olds are gradually decreased. For minimum input voltage
(1.8V) they are typically 1.7A and 2.3A respectively. See
the Typical Performance Characteristics and the Inductor
Selection section for information about the inductor value
for maximum output current capability.
3536fa