ltm4612 Linear Technology Corporation, ltm4612 Datasheet - Page 10

• Manufacturer Part Number: ltm4612
• Description: Ultralow Noise 36vin, 15vout, 5a, Dc/dc ?module
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTM4612
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 specifi c external capacitor
requirements for a particular application.
V
There are restrictions in the maximum V
down ratio that can be achieved for a given input voltage.
These constraints are shown in the Typical Performance
Characteristic curve labeled “V
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 V
resistor, R
the output voltage.
Table 1. R
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
R
Calculate V
Where %V
and V
Where R
pin to ground.
10
R
V
IN
PGM
FB
OUT
R
V
V
to V
OUT
(kΩ)
OUT MARGIN
PGM
(V)
OUT(MARGIN)
resistor on the MPGM pin programs the current.
OUT
(
=
FB
PGM
=
FB
22.1
0 6
OUT
3.3
Standard 1% Resistor Values vs V
OUT(MARGIN)
V
0 6
.
Stepdown Ratios
, from the V
OUT
.
is the resistor value to place on the MPGM
V
is the percentage of V
V
)
•
13.7
=
•
100
5
is the margin quantity in volts:
V
%
OUT MARGIN
100
R
k R
V
1 18
OUT
OUT
FB
11
+
:
6
.
(
FB
and V
pin to the SGND pin programs
FB
•
V
8.06
V
8
OUT
)
FB
IN
•
6.34
pins together. Adding a
10
10
to V
OUT
k
IN
5.23
OUT
OUT
12
to be margined,
and V
Step-Down
4.42
14
OUT
4.12
step
15
The output margining will be ± margining of the value.
This is controlled by the MARG0 and MARG1 pins. See
the truth table below:
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 effi 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.
Figure 3. Inductor Current Ripple vs Output Voltage
MARG1
Figure 2. Operating Frequency vs Output Voltage
HIGH
HIGH
LOW
LOW
1200
1000
800
600
400
200
3.5
3.0
2.5
2.0
1.5
1.0
0.5
2
2
4
4
6
6
MARG0
V
V
8
V
HIGH
HIGH
8
LOW
LOW
OUT
IN
OUT
= 20V
(V)
(V)
10
10
12
12
V
IN
V
= 36V
IN
14
14
= 28V
4612 F02
4612 F03
MARGIN DOWN
16
16
NO MARGIN
MARGIN UP
NO MARGIN
MODE
4612f