LTM4607 Datasheet, PDF (13/24 Pages) Linear Technology – 36VIN, 24VOUT High Effi ciency Buck-Boost DC/DC μModule

LTM4607
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_CC and EXTV_CC
An internal P-channel low dropout regulator produces 6V
at the INTV_CC pin from the V_IN supply pin. INTV_CC powers
the control chip and internal circuitry within the module.
The LTM4607 also provides the external supply voltage pin
EXTV_CC. When the voltage applied to EXTV_CC rises above
5.7V, the internal regulator is turned off and an internal
switch connects the EXTV_CC pin to the INTV_CC pin thereby
supplying internal power. The switch remains close as long
as the voltage applied to EXTV_CC remains above 5.5V. This
allows the MOSFET driver and control power to be derived
from the output when (5.7V < V_OUT < 7V) and from the
internal regulator when the output is out of regulation (start-
up, short-circuit). If more current is required through the
EXTV_CC switch than is specified, an external Schottky diode
can be interposed between the EXTV_CC and INTV_CC pins.
Ensure that EXTV_CC ≤ V_IN.
The following list summarizes the three possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause INTV_CC
to be powered from the internal 6V regulator at the cost
of a small efficiency penalty.
2. EXTV_CC connected directly to V_OUT (5.7V < V_OUT <
7V). This is the normal connection for a 6V regulator and
provides the highest efficiency.
3. EXTV_CC connected to an external supply. If an external
supply is available in the 5.5V to 7V range, it may be
used to power EXTV_CC provided it is compatible with the
MOSFET gate drive requirements.
Thermal Considerations and Output Current Derating
In different applications, LTM4607 operates in a variety
of thermal environments. The maximum output current is
limited by the environmental thermal condition. Sufficient
cooling should be provided to ensure reliable operation.
When the cooling is limited, proper output current de-
rating is necessary, considering ambient temperature,
airflow, 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_JA for
the module. Column designation delineates between no
heatsink, and a BGA heatsink. 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_JA of the condition. A complete explanation of the thermal
characteristics is provided in the thermal application note
for the LTM4607.
DESIGN EXAMPLES
Buck Mode Operation
As a design example, use input voltage V_IN = 12V to 36V,
V_OUT = 12V and f = 400kHz.
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_FB from V_FB
pin to ground should be chosen as:
R_FB
=
0.8V
V_OUT
• 100k
− 0.8V
≈
7.15k
To choose a proper inductor, we need to know the current
ripples 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
4607f
13