LTM4603 LINER [Linear Technology], LTM4603 Datasheet - Page 12

• Manufacturer Part Number: LTM4603
• Description: 36VIN, 34VOUT High Efficiency Buck-Boost DC/DC
• Manufacturer: LINER [Linear Technology]

applications inForMation
LTM4609
In buck mode, the allowed maximum average load cur-
rent is:
The maximum current sensing R
mode is:
The maximum current sensing R
mode is:
A 20% to 30% margin on the calculated sensing resistor
is usually recommended. Please refer to Table 3 for the
recommended 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
the internal reference and the output voltage. The total
soft-start time can be calculated as:
When the RUN pin falls below 1.6V, then soft-start pin
is reset to allow for proper soft-start control when the
regulator is enabled again. Current foldback and force
continuous mode are disabled during the soft-start pro-
cess. 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.
12
I
R
R
t
2 •I
OUT(MAX,BUCK)
SOFTSTART
SENSE(MAX,BOOST)
SENSE(MAX,BUCK)
OUT(MAX,BOOST)
2 • 160mV • V
=
2.4V • C
=
1.7µA
R
130mV
=
SENSE
=
• V
2 •I
SS
OUT
OUT(MAX,BUCK)
IN
+
+ ΔI
2 • 130mV
ΔI
2
SENSE
SENSE
L
L
• V
value for the boost
IN
value for the buck
– ΔI
L
Run Enable
The RUN pin is used to enable the power module. The pin
can be driven with a logic input, not to 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:
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.
COMP Pin
This pin is the external compensation pin. The module
has already been internally compensated for most output
voltages. A spice model is available for other control loop
optimization.
Fault Conditions: Current Limit and Overcurrent
Foldback
LTM4609 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 LTM4609 provides foldback current limiting. If the
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
V _UVLO =
R1+ R2
R2
• 1.6V
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