LTM4612 Ver la hoja de datos (PDF) - Linear Technology
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LTM4612 Datasheet PDF : 24 Pages
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LTM4612
APPLICATIONS INFORMATION
The typical LTM4612 application circuit is shown in
Figure 18. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 2 for speciï¬c external capacitor
requirements for a particular application.
VIN to VOUT Stepdown Ratios
There are restrictions in the maximum VIN and VOUT step
down ratio that can be achieved for a given input voltage.
These constraints are shown in the Typical Performance
Characteristic curve labeled “VIN to VOUT Step-Down
Ratio.†Note that additional thermal derating may be ap-
plied. See the Thermal Considerations and Output Current
Derating section in this data sheet.
Output Voltage Programming and Margining
The PWM controller has an internal 0.6V reference voltage.
As shown in the Block Diagram, a 100k internal feedback
resistor connects the VOUT and VFB pins together. Adding a
resistor, RFB, from the VFB pin to the SGND pin programs
the output voltage.
VOUT
=
0.6V
•
100k + RFB
RFB
Table 1. RFB Standard 1% Resistor Values vs VOUT
VOUT (V) 3.3 5
6
8 10 12 14 15
RFB (kΩ) 22.1 13.7 11 8.06 6.34 5.23 4.42 4.12
The MPGM pin programs a current that when multiplied
by an internal 10k resistor sets up the 0.6V reference ±
offset for margining. A 1.18V reference divided by the
RPGM resistor on the MPGM pin programs the current.
Calculate VOUT(MARGIN):
VOUT(MARGIN)
=
%VOUT
100
• VOUT
Where %VOUT is the percentage of VOUT to be margined,
and VOUT(MARGIN) is the margin quantity in volts:
RPGM
=
VOUT
0.6V
•
1.18V
VOUT(MARGIN)
• 10k
Where RPGM is the resistor value to place on the MPGM
pin to ground.
10
The output margining will be ± margining of the value.
This is controlled by the MARG0 and MARG1 pins. See
the truth table below:
MARG1
LOW
LOW
HIGH
HIGH
MARG0
LOW
HIGH
LOW
HIGH
MODE
NO MARGIN
MARGIN UP
MARGIN DOWN
NO MARGIN
Operating Frequency
The operating frequency of the LTM4612 is optimized to
achieve the compact package size and the minimum
output ripple voltage while still keeping high efï¬ciency.
As shown in Figure 2, the frequency is linearly increased
with larger output voltages to keep the low output cur-
rent ripple. Figure 3 shows the inductor current ripple ΔI
with different output voltages. In most applications, no
additional frequency adjusting is required.
1200
1000
800
600
400
200
24
6 8 10 12 14 16
VOUT (V)
4612 F02
Figure 2. Operating Frequency vs Output Voltage
3.5
3.0
VIN = 36V
2.5
2.0
VIN = 20V
VIN = 28V
1.5
1.0
0.5
2 4 6 8 10 12 14 16
VOUT (V)
4612 F03
Figure 3. Inductor Current Ripple vs Output Voltage
4612f
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