LTM4605EV View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4605EV' PDF : 24 Pages View PDF
LTM4605
APPLICATIONS INFORMATION
RSENSE Selection and Maximum Output Current
RSENSE is chosen based on the required inductor current.
Since the maximum inductor valley current at buck mode
is much lower than the inductor peak current at boost
mode, different sensing resistors are suggested to use
in buck and boost modes.
The current comparator threshold sets the peak of the
inductor current in boost mode and the maximum inductor
valley current in buck mode. In boost mode, the allowed
maximum average load current is:
IOUT(MAX,BOOST)
=
⎛
⎝⎜
160mV
RSENSE
−
ΔIL
2
⎞
⎠⎟
•
VIN
VOUT
where ΔIL is peak-to-peak inductor ripple current.
In buck mode, the allowed maximum average load cur-
rent is:
IOUT(MAX,BUCK)
=
130mV
RSENSE
+
ΔIL
2
The maximum current sensing RSENSE value for the boost
mode is:
RSENSE(MAX,BOOST) =
2 • 160mV • VIN
2 •IOUT(MAX,BOOST) • VOUT + ΔIL • VIN
The maximum current sensing RSENSE value for the buck
mode is:
RSENSE(MAX,BUCK)
=
2
2 • 130mV
• IOUT(MAX,BUCK)
–
ΔIL
A 20% to 30% margin on the calculated sensing resistor is
usually recommended. Please refer to Table 3 for the rec-
ommended sensing resistors for different applications.
Soft-Start
The SS pin provides a means to soft-start the regulator.
A capacitor on this pin will program the ramp rate of the
output voltage. A 1.7μA current source will charge up the
external soft-start capacitor. This will control the ramp of
12
the internal reference and the output voltage. The total
soft-start time can be calculated as:
tSOFTSTART
=
2.4V • CSS
1.7µA
When the RUN pin falls below 1.6V, then soft-start pin is
reset to allow for proper soft-start control when the regula-
tor is enabled again. Current foldback and force continuous
mode are disabled during the soft-start process. The soft-
start function can also be used to control the output ramp
up time, so that another regulator can be easily tracked.
Do not apply more than 6V to the SS pin.
Run Enable
The RUN pin is used to enable the power module. The pin
can be driven with a logic input, and not exceed 6V.
The RUN pin can also be used as an undervoltage lockout
(UVLO) function by connecting a resistor from the input
supply to the RUN pin. The equation:
V _UVLO = R + 100k • 1.6V
100k
Power Good
The PGOOD pin is an open drain pin that can be used to
monitor valid output voltage regulation. This pin monitors
a ±7.5% window around the regulation point, and tracks
with margining.
COMP Pin
This pin is the external compensation pin. The module
has already been internally compensated for most output
voltages. A spice model will be provided for other control
loop optimization.
Fault Conditions: Current Limit and Overcurrent
Foldback
LTM4605 has a current mode controller, which inherently
limits the cycle-by-cycle inductor current not only in steady
state operation, but also in transient. Refer to Table 3.
To further limit current in the event of an overload condi-
tion, the LTM4605 provides foldback current limiting. If the
4605fa
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