Datasheet

LTM8055/LTM8055-1
15
Rev C
For more information www.analog.com
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/LTM8055-1 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 condi
-
tions, which
can be useful when designing to EMI or output
noise specifications. 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/LTM8055-1 transitions
from buck, buck-boost or boost operating modes, espe
-
cially at
lighter loads. In such a case, it can be desirable
to operate 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 cur
-
rent falls
to this level, the LL pin will pull the MODE pin
down, putting the LTM8055/LTM8055-1 in discontinuous
mode, preventing reverse current 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/LTM8055-1 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 cur-
rent 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/LTM8055-1. The LTM8055/
LTM8055-1 is nevertheless 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.
APPLICATIONS INFORMATION
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
Figure 5b. This Comparator Circuit Turns Off the LTM8055/
LTM8055-1 if the Input Rises Above a Predetermined
Threshold. When the LTM8055/LTM8055-1 Turns Off, the
Energy Stored in the Internal Inductor Will Raise V
IN
a Small
Amount Above the Threshold
V
IN
ZENER
DIODE
R
Q
8055 F05a
LTM8055/
LTM8055-1
LOAD
CURRENT
GND
V
OUT
SOURCING
LOAD
V
IN
RUN
8055 F05b
10µF
LTM8055/
LTM8055-1
LOAD
CURRENT
GND
V
OUT
SOURCING
LOAD
EXTERNAL
REFERENCE
VOLTAGE
+
–
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.