參數(shù)資料
型號(hào)： LTC3816EFE#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PDSO38
封裝： LEAD FREE, PLASTIC, TSSOP-38
文件頁(yè)數(shù)： 29/44頁(yè)
文件大?。?/td> 753K
代理商： LTC3816EFE#TRPBF
LTC3816
3816f
PULSE
GENERATOR
0V TO 10V
100Hz, 1% TO 5%
DUTY CYCLE
LTC3816
50
10k
3816 F20
LOCATE CLOSE TO THE OUTPUT
RLOAD
RENESAS RJK0305DPB
OR EQUIVALENT
VOUT
Figure 20. Transient Load Generator PC Board
applicaTions inForMaTion
Conveniently, the typical probe tip ground clip is spaced
just right to span the leads of a typical output capacitor. In
general,itisbesttotakethismeasurementwiththe20MHz
bandwidth limit on the oscilloscope turned on to limit high
frequencynoise.Notethatmicroprocessormanufacturers
typically specify ripple ≤20MHz, as energy above 20MHz
is generally radiated and not conducted and will not affect
the load even if it appears at the output capacitor.
Now that we know how to measure the signal, we need to
have something to measure. The ideal situation is to use
the actual load for the test, and switch it on and off while
watching the output. If this isn’t convenient, a current
step generator is needed. This generator needs to be able
to turn on and off in nanoseconds to simulate a typical
switching logic load, so stray inductance and long clip
leads between the LTC3816 and the transient generator
must be minimized.
Figure 20 shows an example of a simple transient gen-
erator. Be sure to use a noninductive resistor as the load
element—many power resistors use an inductive spiral
pattern and are not suitable for use here. A simple solution
is to take ten 1/4W film resistors and wire them in parallel
to get the desired value. This gives a noninductive resis-
tive load which can dissipate 2.5W continuously or 50W
if pulsed with a 5% duty cycle, enough for most LTC3816
circuits. Solder the MOSFET and the resistor(s) as close
to the output of the LTC3816 circuit as possible and set
up the signal generator to pulse at a 100Hz rate with a 5%
duty cycle. This pulses the LTC3816 with 500s transients
10ms apart, adequate for viewing the entire transient
recovery time for both positive and negative transitions
while keeping the load resistor cool.
A DESIGN EXAMPLE
As a design example, consider an IMVP-6.5 application
with inductor DCR current sense (see the last page
schematic) and the following requirements: assume VIN
= 12V (nominal), VIN = 24V (maximum), VOUT = 0.75V,
VOUT (minimum) = 0.725V, ILOAD(MAX) = 27A, ILOAD(MIN)
= 1.5A, AVP = –3mV/A, fOSC = 400kHz, VIMON = 1.0V.
For the input and output conditions given above, the
steady-state minimum on-time for this application at
VIN = 24V is approximately:
t
V
f
V
kH
ON MIN
OUT MIN
IN MAX
OSC
(
)
(
)
(
)
.
=
0 725
24 400 zz
ns
= 75 5
.
This is much longer than the LTC3816 minimum on-time.
To program the 400kHz operation, float the RFREQ pin.
The inductance value is chosen first based on a 20% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage:
L
V
f
I
V
OUT
L MAX
OUT
IN MAX
=
=
–
.
(
)
(
)
1
0 75
4000
0 2 27
1
0 75
24
0 33
kHz
A
V
H
.
–
.
=
A commonly available 0.33H inductor is chosen. This
results in 5.5A of ripple current. The peak inductor cur-
rent is the maximum DC load current plus one-half the
ripple current, or:
I
A
L PEAK
(
)
.
.
=
+
=
27
1
2
5 5
29 75
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