參數(shù)資料
型號(hào)： LTC3731HUH#PBF
廠(chǎng)商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 5 A SWITCHING CONTROLLER, 750 kHz SWITCHING FREQ-MAX, PQCC32
封裝： 5 X 5 MM, LEAD FREE, PLASTIC, MO-220WHHD, QFN-32
文件頁(yè)數(shù)： 19/34頁(yè)
文件大?。?/td> 427K
代理商： LTC3731HUH#PBF
LTC3731H
3731Hfb
applicaTions inForMaTion
The worst-case input RMS ripple current for a single stage
design peaks at twice the value of the output voltage. The
worst-caseinputRMSripplecurrentforatwostagedesign
results in peaks at 1/4 and 3/4 of the input voltage, and
the worst-case input RMS ripple current for a three stage
design results in peaks at 1/6, 1/2, and 5/6 of the input
voltage. The peaks, however, are at ever decreasing levels
with the addition of more phases. A higher effective duty
factor results because the duty factors “add” as long as
the currents in each stage are balanced. Refer to AN19
for a detailed description of how to calculate RMS current
for the single stage switching regulator.
Figure 6 illustrates the RMS input current drawn from the
input capacitance versus the duty cycle as determined by
the ratio of input and output voltage. The peak input RMS
current level of the single phase system is reduced by 2/3
in a 3-phase solution due to the current splitting between
the three stages.
The output ripple current is reduced significantly when
compared to the single phase solution using the same
inductance value because the VOUT/L discharge currents
term from the stages that has their bottom MOSFETs on
subtract current from the (VCC – VOUT)/L charging current
resulting from the stage which has its top MOSFET on.
The output ripple current for a 3-phase design is:
IP-P = ( )( )(
)
>
V
f L
DC V
V
OUT
IN
OUT
1 3
3
–
The ripple frequency is also increased by three, further re-
ducing the required output capacitance when VCC < 3VOUT
as illustrated in Figure 6.
The addition of more phases, by phase locking addi-
tional controllers, always results in no net input or output
ripple at VOUT/VIN ratios equal to the number of stages
implemented. Designing a system with multiple stages
close to the VOUT/VIN ratio will significantly reduce the
ripplevoltageattheinputandoutputsandtherebyimprove
efficiency, physical size and heat generation of the overall
switching power supply. Refer to Application Note 77 for
more information on Polyphase circuits.
Efficiency Calculation
To estimate efficiency, the DC loss terms include the input
andoutputcapacitorESR,eachMOSFETRDS(ON),inductor
resistanceRL,thesenseresistanceRSENSEandtheforward
dropoftheSchottkyrectifierattheoperatingoutputcurrent
and temperature. Typical values for the design example
given previously in this data sheet are:
Main MOSFET RDS(ON) = 7mΩ (9mΩ at 90°C)
Sync MOSFET RDS(ON) = 7mΩ (9mΩ at 90°C)
CINESR = 20mΩ
COUTESR = 3mΩ
RL = 2.5mΩ
RSENSE = 3mΩ
VSCHOTTKY = 0.8V at 15A (0.7V at 90°C)
VOUT = 1.3V
VIN = 12V
IMAX = 45A
d = 0.5%°C (MOSFET temperature coefficient)
N = 3
f = 400kHz
The main MOSFET is on for the duty factor VOUT/VIN and
the synchronous MOSFET is on for the rest of the period
or simply (1 – VOUT/VIN). Assuming the ripple current is
small, the AC loss in the inductor can be made small if
a good quality inductor is chosen. The average current,
IOUT, is used to simplify the calculations. The equation
below is not exact but should provide a good technique
for the comparison of selected components and give a
result that is within 10% to 20% of the final application.
Determining the MOSFETs’ die temperature may require
iterative calculations if one is not familiar with typical
performance. A maximum operating junction temperature
of 90° to 100°C for the MOSFETs is recommended for
high reliability applications.
Common output path DC loss:
P
N
I
N
R
C
Lo
COMPATH
MAX
L
SENSE
OUTESR
≈
+
(
)+
2
sss
This totals 3.7W + COUTESR loss.
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