LTM4609 LINER [Linear Technology], LTM4609 Datasheet - Page 13

• Manufacturer Part Number: LTM4609
• Description: 36VIN, 34VOUT High Effi ciency Buck-Boost DC/DC ?Module
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
pin voltage <1.6V). In this mode, the onboard 6V linear
regulator can provide power to keep-alive functions such
as a keyboard controller.
INTV
An internal P-channel low dropout regulator produces 6V
at the INTV
the control chip and internal circuitry within the module.
The LTM4609 also provides the external supply voltage pin
EXTV
5.7V, the internal regulator is turned off and an internal
switch connects the EXTV
supplying internal power. The switch remains close as long
as the voltage applied to EXTV
allows the MOSFET driver and control power to be derived
from the output when (5.7V < V
internal regulator when the output is out of regulation (start-
up, short-circuit). If more current is required through the
EXTV
can be interposed between the EXTV
Ensure that EXTV
The following list summarizes the three possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
Thermal Considerations and Output Current Derating
In different applications, LTM4609 operates in a variety
of thermal environments. The maximum output current is
limited by the environmental thermal condition. Suffi cient
cooling should be provided to ensure reliable operation.
to be powered from the internal 6V regulator at the cost
of a small effi ciency penalty.
This is the normal connection for a 6V regulator and
provides the highest effi ciency.
supply is available in the 5.5V to 7V range, it may be
used to power EXTV
the MOSFET gate drive requirements.
CC
CC
CC
. When the voltage applied to EXTV
and EXTV
switch than is specifi ed, an external Schottky diode
CC
CC
CC
left open (or grounded). This will cause INTV
connected directly to V
connected to an external supply. If an external
CC
pin from the V
CC
:
CC
CC
≤ V
CC
IN
CC
.
provided it is compatible with
IN
pin to the INTV
CC
supply pin. INTV
remains above 5.5V. This
OUT
OUT
CC
< 7V) and from the
(5.7V < V
and INTV
CC
CC
rises above
pin thereby
CC
OUT
CC
powers
< 7V).
pins.
CC
When the cooling is limited, proper output current de-
rating is necessary, considering ambient temperature,
airfl ow, input/output condition, and the need for increased
reliability.
The power loss curves in Figures 5 and 6 can be used
in coordination with the load current derating curves in
Figures 7 to 14 for calculating an approximate θ
the module. Column designation delineates between no
heat sink, and a BGA heat sink. Each of the load current
derating curves will lower the maximum load current as
a function of the increased ambient temperature to keep
the maximum junction temperature of the power module
at 115°C allowing a safe margin for the maximum operat-
ing temperature below 125°C. Each of the derating curves
and the power loss curve that corresponds to the correct
output voltage can be used to solve for the approximate
θ
characteristics is provided in the thermal application note
for the LTM4609.
DESIGN EXAMPLES
Buck Mode Operation
As a design example, use input voltage V
V
Set the PLLFLTR pin at 2.4V or more for 400kHz frequency
and connect FCB to ground for continuous current mode
operation. If a divider is used to set the frequency as shown
in Figure 16, the bottom resistor R3 is recommended not
to exceed 1kΩ.
To set the output voltage at 12V, the resistor R
pin to ground should be chosen as:
To choose a proper inductor, we need to know the current
ripple at different input voltages. The inductor should be
chosen by considering the worst case in the practical
operating region. If the maximum output power P is 120W
JA
OUT
R
of the condition. A complete explanation of the thermal
FB
= 12V and ƒ = 400kHz.
=
V
0.8V • 100k
OUT
− 0.8V
≈ 7.15k
LTM4609
IN
= 12V to 36V,
FB
from V
13
JA
4609fa
for
FB