LTM8055_15 Datasheet, PDF (15/24 Pages) Linear Technology – 36VIN, 8.5A Buck-Boost Module Regulator

LTM8055
Applications Information
LOAD
CURRENT
V_IN
V_OUT
ZENER
DIODE
LTM8055
Q
GND
SOURCING
LOAD
R
8055 F05a
Figure 5a. The MOSFET Q Dissipates Momentary Energy to
GND. The Zener Diode and Resistor Are Chosen to Ensure That
the MOSFET Turns On Above the Maximum V_IN Voltage Under
Normal Operation
V_IN
V_OUT
LTM8055
LOAD
CURRENT
RUN
GND
SOURCING
LOAD
10µF
–
+
EXTERNAL
REFERENCE
VOLTAGE
8055 F05b
Figure 5b. This Comparator Circuit Turns Off the LTM8055 if
the Input Rises Above a Predetermined Threshold. When the
LTM8055 Turns Off, the Energy Stored in the Internal Inductor
Will Raise V_IN a Small Amount Above the Threshold
Switching Mode
The MODE pin allows the user to select either discontinuous
mode or forced continuous mode switching operation. In
forced continuous mode, the LTM8055 will not skip cycles,
even when the internal inductor current falls to zero or even
reverses direction. This has the advantage of operating at
the same fixed frequency for all load conditions, which can
be useful when designing to EMI or output noise speci-
fications. Forced continuous mode, however, uses more
current at light loads, and allows current to flow from the
load back into the input if the output is raised above the
regulation point. This reverse current can raise the input
voltage and be hazardous if the input is allowed to rise
uncontrollably. Please refer to Input Precautions in this
section for a discussion of this behavior.
Forced continuous operation may provide improved
output regulation when the LTM8055 transitions from
buck, buck-boost or boost operating modes, especially at
lighter loads. In such a case, it can be desirable to oper-
ate in forced continuous mode except when the internal
inductor current is about to reverse. If so, apply a current
sense resistor between V_OUT and I_OUT and tie the LL and
MODE pins together. The LL pin is low when the current
through the output sense resistor is about one-tenth the
full-scale maximum. When the output current falls to this
level, the LL pin will pull the MODE pin down, putting the
LTM8055 in discontinuous mode, preventing reverse cur-
rent from flowing from the output to the input. In the case
where MODE and LL are tied together, a small capacitor
(~0.1µF) from these pins to GND may improve the light
load transient response by delaying the transition from
the discontinuous to forced continuous switching modes.
MODE may be tied to GND for the purpose of blocking
reverse current if no output current sense resistor is used.
FB Resistor Divider and Load Regulation
The LTM8055 regulates its FB pin to 1.2V, using a resistor
divider to sense the output voltage. The location at which
the output voltage is sensed affects the load regulation.
If there is a current sense resistor between V_OUT and
I_OUT, and the output is sensed at V_OUT, the voltage at the
load will drop by the value of the current sense resistor
multiplied by the output current. If the output voltage can
be sensed at I_OUT, the load regulation may be improved.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8055. The LTM8055 is neverthe-
less a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 6
for a suggested layout. Ensure that the grounding and
heat sinking are acceptable.
A few rules to keep in mind are:
1. Place the R_FB and R_T resistors as close as possible to
their respective pins.
For more information www.linear.com/LTM8055
8055f
15