LTM4609_15 Datasheet, PDF (9/28 Pages) Linear Technology – 36VIN, 34VOUT High Efficiency Buck-Boost DC/DC Module Regulator

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
controller, ultralow R_DS(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 operating
frequency of the LTM4609 can be adjusted 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 switch-
ing frequency is 400kHz.
The Burst Mode^® and skip-cycle mode operations can be
enabled at light loads to improve efficiency, while the forced
continuous mode and discontinuous mode operations are
used for constant frequency applications. Foldback current
limiting is activated in an overcurrent condition as V_FB
drops. Internal overvoltage and undervoltage compara-
tors 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 EXTV_CC 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 end of the input voltage range.
Applications Information
The typical LTM4609 application circuit is shown in Fig-
ure 18. External component selection is primarily deter-
mined 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 reference voltage.
As shown in the Block Diagram, a 100k internal feedback
resistor connects V_OUT and V_FB pins together. Adding a
resistor R_FB from the V_FB pin to the SGND pin programs
the output voltage:
V_OUT
=
0.8V
•
100k +R_FB
R_FB
Table 1. R_FB Resistor (0.5%) vs Output Voltage
V_OUT 0.8V 1.5V 2.5V 3.3V 5V 6V
R_FB Open 115k 47.5k 32.4k 19.1k 15.4k
V_OUT 10V 12V 15V 16V 20V 24V
R_FB 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 PLLFLTR is left open, the PLLFLTR pin goes low,
forcing 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 lower. 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
4609ff
For more information www.linear.com/LTM4609
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