LTM4605
13
4605fd
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applications inForMation
output voltage falls by more than 70%, then the maximum
output current is progressively lowered to about 30% of
its full current limit value for boost mode and about 40%
for buck mode.
Standby Mode (STBYMD)
The standby mode (STBYMD) pin provides several choices
for start-up and standby operational modes. If the pin is
pulled to ground, the SS pin is internally pulled to ground,
preventing start-up and thereby providing a single control
pin for turning off the controller. If the pin is left open or
decoupled with a capacitor to ground, the SS pin is internally
provided with a starting current, permitting external control
for turning on the controller. If the pin is connected to a
voltage greater than 1.25V, the internal regulator (INTV
CC
)
will be on even when the controller is shut down (RUN
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 LTM4605 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 closed 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, the LTM4605 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 derating
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 12 for calculating an approximate θ
JA
for
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 maximum. This will allow a safe margin to work
at the maximum operating 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.
DESIGN EXAMPLES
Buck Mode Operation
As a design example, use input voltage V
IN
= 12V to 20V,
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 14, the bottom resistor R3 is recommended not
to exceed 1k.
