LTM4630Y Просмотр технического описания (PDF) - Linear Technology
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LTM4630Y Datasheet PDF : 34 Pages
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LTM4630
APPLICATIONS INFORMATION
The typical LTM4630 application circuit is shown in Fig-
ure 23. External component selection is primarily deter-
mined by the maximum load current and output voltage.
Refer to Table 4 for specific external capacitor requirements
for particular applications.
VIN to VOUT Step-Down Ratios
There are restrictions in the maximum VIN and VOUT step-
down ratio that can be achieved for a given input voltage.
Each output of the LTM4630 is capable of 98% duty cycle,
but the VIN to VOUT minimum dropout is still shown as a
function of its load current and will limit output current
capability related to high duty cycle on the top side switch.
Minimum on-time tON(MIN) is another consideration in
operating at a specified duty cycle while operating at a
certain frequency due to the fact that tON(MIN) < D/fSW,
where D is duty cycle and fSW is the switching frequency.
tON(MIN) is specified in the electrical parameters as 90ns.
Output Voltage Programming
The PWM controller has an internal 0.6V reference voltage.
As shown in the Block Diagram, a 60.4kΩ internal feedback
resistor connects between the VOUTS1 to VFB1 and VOUTS2
to VFB2. It is very important that these pins be connected
to their respective outputs for proper feedback regulation.
Overvoltage can occur if these VOUTS1 and VOUTS2 pins are
left floating when used as individual regulators, or at least
one of them is used in paralleled regulators. The output
voltage will default to 0.6V with no feedback resistor on
either VFB1 or VFB2. Adding a resistor RFB from VFB pin to
GND programs the output voltage:
VOUT
=
0.6V
•
60.4k +RFB
RFB
Table 1. VFB Resistor Table vs Various Output Voltages
VOUT
0.6V
1.0V
1.2V
1.5V
RFB
Open
90.9k
60.4k
40.2k
1.8V
30.2k
For parallel operation of multiple channels the same feed-
back setting resistor can be used for the parallel design.
This is done by connecting the VOUTS1 to the output as
shown in Figure 2, thus tying one of the internal 60.4k
resistors to the output. All of the VFB pins tie together with
one programming resistor as shown in Figure 2.
In parallel operation, the VFB pins have an IFB current of 20nA
maximum each channel. To reduce output voltage error due
to this current, an additional VOUTS pin can be tied to VOUT,
and an additional RFB resistor can be used to lower the total
Thevenin equivalent resistance seen by this current. For
example in Figure 2, the total Thevenin equivalent resistance
of the VFB pin is (60.4k//RFB), which is 30.2k where RFB is
equal to 60.4k for a 1.2V output. Four phases connected
in parallel equates to a worse case feedback current of
4 • IFB = 80nA maximum. The voltage error is 80nA • 30.2k
= 2.4mV. If VOUTS2 is connected, as shown in Figure 2, to
VOUT, and another 60.4k resistor is connected from VFB2 to
ground, then the voltage error is reduced to 1.2mV. If the
voltage error is acceptable then no additional connections
are necessary. The onboard 60.4k resistor is 0.5% accurate
and the VFB resistor can be chosen by the user to be as
accurate as needed. All COMP pins are tied together for
current sharing between the phases. The TRACK/SS pins
can be tied together and a single soft-start capacitor can
be used to soft-start the regulator. The soft-start equation
will need to have the soft-start current parameter increased
by the number of paralleled channels. See Output Voltage
Tracking section.
COMP1 LTM4630
COMP2
VOUT1
VOUT2
TRACK1
TRACK2
60.4k
60.4k
VOUTS1
VOUTS2
VFB1
VFB2
0.1µF
COMP1 LTM4630
COMP2
VOUT1
VOUT2
TRACK1
TRACK2
60.4k
60.4k
VOUTS1
VOUTS2
VFB1
VFB2
4630 F02
4 PARALLELED OUTPUTS
FOR 1.2V AT 70A
OPTIONAL CONNECTION
OPTIONAL
RFB
60.4k
USE TO LOWER
TOTAL EQUIVALENT
RESISTANCE TO LOWER
IFB VOLTAGE ERROR
RFB
60.4k
Figure 2. 4-Phase Parallel Configurations
For more information www.linear.com/LTM4630
4630fa
11
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