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| 型号： | LTC3569EFE#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.5 A SWITCHING REGULATOR, 3000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 4.40 MM, LEAD FREE, PLASTIC, TSSOP-16 |
| 文件页数： | 7/26页 |
| 文件大小： | 468K |
| 代理商： | LTC3569EFE#PBF |
LTC3569
15
3569fd
APPLICATIONS INFORMATION
Setting the Output Voltages
The LTC3569 develops independent internal reference
voltages for each of the feedback pins. These reference
voltages are programmed from 0.8V down to 0.425V in
–25mV increments by toggling the appropriate EN pin.
The output voltage is set by a resistive divider according
to the following formula (refer to Figure 9 for resistor
designations):
VOUT1 = VREF1(1 + R1/R2),
where VREF1 is programmed by toggling the EN1 pin.
VOUT2 = VREF2(1 + R3/R4),
where VREF2 is programmed by toggling the EN2 pin.
VOUT3 = VREF3(1 + R5/R6),
where VREF3 is programmed by toggling the EN3 pin.
Keeping the current small (<5A) in these resistors
maximizes efficiency, but making the current too small
may allow stray capacitance to cause noise problems and
reduce the phase margin of the error amp loop.
To improve the frequency response, use a feedforward
capacitor, CF, on the order of 20pF across the leading
feedback resistor (R1, R3, and R5). Take care to route
each FB line away from noise sources, such as the induc-
tor or the SW line. Remove the ground plane from below
the FB PCB routes to limit stray capacitance to GND on
these pins.
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current
IL decreases with
higher inductance and increases with higher VIN or VOUT:
IL = VOUT/(fCLKL )(1–VOUT/VIN)
Accepting larger values of
IL allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
IL = 0.3IOUT(MAX), where IOUT(MAX) is the maximum
load current. The largest ripple current
IL occurs at the
maximuminputvoltage.Toguaranteethattheripplecurrent
stays below a specified maximum, choose the inductor
value according to the following equation:
L = VOUT/(fCLKIL)(1 – VOUT/VIN(MAX))
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the peak inductor current falls below a level set by
the burst clamp. Lower inductor values result in higher
ripple current which causes this to occur at lower load
currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values increase the burst frequency and
reduces efficiency.
Choose an inductor with a DC current rating at least 1.5
times larger than the maximum load current to ensure
that the inductor core does not saturate during normal
operation.Ifanoutputshort-circuitisapossiblecondition,
select an inductor that is rated to handle the maximum
peak current specified for the regulators. To maximize
efficiency, choose an inductor with a low DC resistance;
as power loss in the inductor is due to I2R losses. Where
I2 is the square of the average output current and R is the
ESR of the inductor.
Table 1. Low Profile Inductors
VENDOR/
PART NUMBER
VALUE
(H)
IDC
(APPROX.)
RDC
(Ω)
HEIGHT
(mm)
Wurth
7440430022
744031002
2.2
2.5
2.50
1.45
0.023
0.050
2.80
1.65
MuRata
LQH55PN1R2
LQH55PN2R2
1.2
2.2
2.60
2.10
0.021
0.031
1.85
Toko, DEV518C
1124BS-1R8N
1124BS-2R4M
1.8
2.4
2.70
2.30
0.047
0.054
1.80
EPCOS
B824691152M000
B824691221M000
1.5
2.2
1.70
1.55
0.046
0.065
1.20
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