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LTM4634 データシート(PDF) 13 Page - Linear Technology

部品番号 LTM4634
部品情報  Triple Output 5A/5A/4A Step-Down DC/DC 關Module짰 Regulator
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メーカー  LINER [Linear Technology]
ホームページ  http://www.linear.com
Logo LINER - Linear Technology

LTM4634 データシート(HTML) 13 Page - Linear Technology

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LTM4634
13
4634f
For more information www.linear.com/LTM4634
applicaTions inForMaTion
frequencies optimize efficiency, eliminate minimum on-
timeissuesforlessthan1Voutput,andcontroltheinductor
ripple currents over the input and output voltage ranges.
24Vinputapplicationsthatconverttooutputvoltagesequal
to 5V (VOUT1,2) and up to 12V (VOUT3) will be required to
set the LTM4634 switching frequency to 750kHz. This is
required to maintain less than 60% inductor ripple current
at the higher output voltages. The 750kHz requirement
for these higher output conversions from 24V will limit
outputvoltagesonotherchannelstobenolowerthan1.5V
due to minimum on-time considerations. There is a way
around this issue by taking one of these outputs, either
5V or 12V, and using it as the source for the 0.8V to 1.5V
output. An example circuit is shown in Figure 26. 5V and
12V input conversions on all three channels can be oper-
ated at lower frequencies across the output ranges so that
minimum on-time is not an issue at low output voltages.
The minimum on-time equation on the next page can be
used to verify that no switching frequency is violating this
parameter. The equations for checking IRIPPLE %:
VIN – VOUT
(
)VOUT
L •IRIPPLE • VIN
=FREQ,
IRIPPLE
CH#MaxLoad
=IRIPPLE%
This verifies that the operating frequencies are selected to
limitinductorripplecurrentstobebelow60%ofmaximum
load, where FREQ is selected frequency in Hertz, IRIPPLE
and maximum load current in amps, and L is inductance
in Henrys. Ch1, Ch2 L = 1.5µH, and Ch3 L = 3.3µH. Maxi-
mum load current IOUT1,2 = 5A, and IOUT3 = 4A, therefore
IRIPPLE should try to stay below 2.5A for Ch1, Ch2, and
2A for Ch3, except for 12V output. The efficiency curves
will show the recommended optimal operating frequency
for the different conversions
A DC voltage should be applied to the FREQ/PLLLPF pin
to set the operating frequency when clock synchroniza-
tion is not used. Figure 2 shows the frequency selection
as a function of the applied DC voltage. This can be done
with a voltage divider from the INTVCC (5V) pin to SGND.
A 10k resistor can be selected as the bottom resistor. The
top resistor, RFREQ, can be determined by using equation:
RFREQ =
5V •10k
FREQV
–10k
where FREQV is the voltage at the FREQ/PLLLPF pin in
Figure 2 that corresponds to a particular frequency. See
Figure 25 for an example.
FREQ/PLLLPF PIN VOLTAGE (V)
0
0.5
1
1.5
2
4634 F02
2.5
300
400
500
800
700
200
600
Figure 2. Relationship Between Oscillator Frequency
and Voltage at the FREQ/PLLLPF Pin
Parallel Channel Operation
For outputs that demand more than 5A of load current,
the LTM4634 device can parallel VOUT1 and VOUT2 to sup-
ply 10A of load current. The two channels will operate at
120° of phase shift. The input RMS ripple current can be
calculated using Equation 1. For example, 12V to 1.2V at
10A equates to duty cycle D = 0.1.
ICIN(RMS) =
10A
0.85
• 0.1• 1– 0.1
(
)
ICIN(RMS)=3.5ARMS,use2×22µF16VX5RorX7Rceramic
capacitors rated at 2ARMS each.
The LTM4634 regulators are inherently current mode
controlled devices, so the paralleling of VOUT1 and VOUT2
channels will have good current sharing. This will balance
the thermals in the design. Tie the COMP, VFB, TK/SS
and RUN pins together for these two channels to share
the current evenly. Figure 24 shows a schematic of the
parallel design.
Minimum On-Time
Minimum on-time, tON, is the smallest time duration that
any of the three regulator channels is capable of turning on
the top MOSFET. It is determined by internal timing delays,
and the gate charge required to turn on the top MOSFET.


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