LTM4609V(RevA) データシートの表示（PDF） - Linear Technology
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LTM4609V Datasheet PDF : 24 Pages
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LTM4609
OPERATION
Power Module Description
The LTM4609 is a non-isolated buck-boost DC/DC power
supply. It can deliver a wide range output voltage from
0.8V to 34V over a wide input range from 4.5V to 36V,
by only adding the sensing resistor, inductor and some
external input and output capacitors. It provides precisely
regulated output voltage programmable via one external
resistor. The typical application schematic is shown in
Figure 18.
The LTM4609 has an integrated current mode buck-boost
control, ultralow RDS(ON) FETs with fast switching speed
and integrated Schottky diodes. With current mode control
and internal feedback loop compensation, the LTM4609
module has sufficient stability margins and good transient
performance under a wide range of operating conditions
and with a wide range of output capacitors. The frequency
of LTM4609 can be operated from 200kHz to 400kHz by
setting the voltage on the PLLFLTR pin. Alternatively, its
frequency can be synchronized by the input clock signal
from the PLLIN pin. The typical switching frequency is
400kHz.
The Burst Mode® and skip-cycle mode operations can
be enabled at light loads in the LTM4609 to improve its
efficiency, while the forced continuous mode and discon-
tinuous mode operations are used for constant frequency
applications. Foldback current limiting is activated in an
overcurrent condition as VFB drops. Internal overvoltage
and undervoltage comparators pull the open-drain PGOOD
output low if the output feedback voltage exits the ±7.5%
window around the regulation point. Pulling the RUN pin
below 1.6V forces the controller into its shutdown state.
If an external bias supply is applied on the EXTVCC pin, then
an efficiency improvement will occur due to the reduced
power loss in the internal linear regulator. This is especially
true at the higher input voltage range.
Burst Mode is a registered trademark of Linear Technology Corporation.
APPLICATIONS INFORMATION
The typical LTM4609 application circuit is shown in
Figure 18. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 3 for specific external capacitor
requirements for a particular application.
Output Voltage Programming
The PWM controller has an internal 0.8V±1% reference
voltage. As shown in the Block Diagram, a 100k 0.5%
internal feedback resistor connects VOUT and VFB pins
together. Adding a resistor RFB from the VFB pin to the
SGND pin programs the output voltage:
VOUT
=
0.8V
•
100k + RFB
RFB
Table 1. RFB Resistor (0.5%) vs Output Voltage
VOUT 0.8V 1.5V 2.5V 3.3V 5V 6V
RFB Open 115k 47.5k 32.4k 19.1k 15.4k
VOUT 10V 12V 15V 16V 20V 24V
RFB 8.66k 7.15k 5.62k 5.23k 4.12k 3.4k
8V
11k
30V
2.74k
9V
9.76k
34V
2.37k
Operation Frequency Selection
The LTM4609 uses current mode control architecture at
constant switching frequency, which is determined by the
internal oscillator’s capacitor. This internal capacitor is
charged by a fixed current plus an additional current that
is proportional to the voltage applied to the PLLFLTR pin.
The PLLFLTR pin can be grounded to lower the frequency
to 200kHz or tied to 2.4V to yield approximately 400kHz.
When PLLIN is left open, the PLLFLTR pin goes low, forc-
ing the oscillator to its minimum frequency.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 2. As the operating frequency
increases, the gate charge losses will be higher, thus the
efficiency is low. The maximum switching frequency is
approximately 400kHz.
FREQUENCY SYNCHRONIZATION
The LTM4609 can also be synchronized to an external
source via the PLLIN pin instead of adjusting the voltage on
the PLLFLTR pin directly. The power module has a phase-
4609fa
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