LTM4606(RevB) Просмотр технического описания (PDF) - Linear Technology
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LTM4606 Datasheet PDF : 28 Pages
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LTM4606
Applications Information
The typical LTM4606 application circuit is shown in Fig-
ure 20. External component selection is primarily deter-
mined by the maximum load current and output voltage.
Refer to Table 2 for specific external capacitor requirements
for a particular application.
VIN to VOUT Step-Down Ratios
Under the default frequency, there are restrictions in
the maximum VIN and VOUT step-down ratio that can be
achieved for a given input voltage. These constraints are
caused by the limitation of the minimum on and off time in
the internal switches. Refer to the Frequency Adjustment
section to change the switching frequency and get wider
input and output ranges. See the Thermal Considerations
and Output Current Derating section in this data sheet for
the current restrictions.
Output Voltage Programming and Margining
The PWM controller has an internal 0.6V reference voltage.
As shown in the Block Diagram, a 60.4k 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
60.4k + RFB
RFB
Table 1. RFB Standard 1% Resistor Values vs VOUT
RFB
(kΩ)
Open 60.4
40.2
30.1
25.5
19.1
13.3
8.25
VOUT
(V)
0.6
1.2
1.5
1.8
2
2.5 3.3
5
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 you want to
margin, 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.
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
Input Capacitors and Input EMI Noise Attenuation
The LTM4606 is designed to achieve low input conducted
EMI noise due to the fast switching of turn-on and turn-off.
In the LTM4606, a high frequency inductor is integrated
to the input line for noise attenuation. VD and VIN pins
are available for external input capacitors to form a high
frequency π filter. As shown in Figure 19, the ceramic
capacitor C1 on the VD pins is used to handle most of
the RMS current into the converter, so careful attention
is needed for capacitor C1 selection.
For a buck converter, the switching duty cycle can be
estimated as:
D = VOUT
VIN
4606fb
11
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