LTM4620 Ver la hoja de datos (PDF) - Analog Devices
Número de pieza
componentes Descripción
Fabricante
LTM4620 Datasheet PDF : 40 Pages
| |||
LTM4620
APPLICATIONS INFORMATION
The typical LTM4620 application circuit is shown in
Figure 23. External component selection is primarily
determined 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 LTM4620 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.
See Note 8 in the Electrical Characteristics section for
output current guideline.
Output Voltage Programming
The PWM controller has an internal 0.6V reference volt-
age. 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 feed-
back 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
1.8V
RFB Open 90.9k 60.4k 40.2k 30.2k
2.5V
19.1k
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 connec-
tions 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 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 the Output Voltage
Tracking section.
COMP1 LTM4620
COMP2
VOUT1
VOUT2
TRACK1
TRACK2
60.4k
60.4k
VOUTS1
VOUTS2
VFB1
VFB2
0.1µF
COMP1 LTM4620
COMP2
VOUT1
VOUT2
TRACK1
TRACK2
60.4k
60.4k
VOUTS1
VOUTS2
VFB1
VFB2
4620 F02
4 PARALLELED OUTPUTS
FOR 1.2V AT 50A
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
4620fc
12
For more information www.linear.com/LTM4620
Share Link: 