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參數(shù)資料
| 型號： | LTC3766MPUFD#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 316 kHz SWITCHING FREQ-MAX, PQCC28 |
| 封裝： | 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-28 |
| 文件頁數(shù)： | 24/60頁 |
| 文件大?。?/td> | 607K |
| 代理商： | LTC3766MPUFD#TRPBF |
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LTC3766
30
3766f
APPLICATIONS INFORMATION
transformer. This causes the gain of the internal current
sense amplifier to be reduced, so that the maximum peak
current voltage is increased to approximately 1V. The
current transformer connections are shown in Figure 12.
voltage and comparing it to the output inductor current.
Figure13 shows an example of a well-calibrated current
sense transformer, where the RSENSE voltage has been
scaled by a factor of:
SF
=
NP
RSENSEKCTNS
Becauseofthemagnetizingcurrent,theslopeofthescaled
RSENSE voltage will not exactly match that of the inductor
current. Choose RCAL so that the scaled RSENSE voltage
and the inductor current are identical at the peak.
DCT
CURRENT
TRANSFORMER
RCAL
1k
MAIN
TRANSFORMER
NS
NP
PG
VIN+
VIN–
RSENSE
CFL
IS+
VCC
IS–
3766 F12
GND
LTC3766
Figure 12. Using a Current Sense Transformer
20A
500ns/DIV
3766 F13
INDUCTOR
CURRENT
2A/DIV
SF VRSENSE
2A/DIV
Figure 13. Properly Calibrated Current Transformer
Typically, the current transformer is placed in series with
thepowersupplyfeedtothemaintransformer.Thisreduces
common mode noise, and is generally convenient for the
PCB layout. Use a small filter capacitor, CFL, between 1nF
and 3.3nF, or a time constant for RSENSE CFL of less than
75ns, to eliminate high frequency noise. The diode DCT
is needed to allow the core of the current transformer to
properly reset.
When using a current transformer, set the value of RSENSE
using:
RSENSE =
0.73V
KCTILIM(AVG)
NP
NS
where NP/NS is the turns ratio of the main transformer,
KCT is the current gain of the transformer, and ILIM(AVG) is
the average current limit desired. For most applications,
a current transformer turns ratio of 1:100 is suitable
(KCT = 0.01).
The resistor RCAL is added to compensate for the ef-
fects of the magnetizing current in the main and current
sense transformers, both of which cause the voltage on
RSENSE to be somewhat higher than expected (2% to
8%). Typically, RCAL is in the range of 1.5k to 5k. For the
highest possible accuracy, the value of RCAL should be
adjusted to calibrate the current sensing at full load and
nominal input voltage by carefully measuring the RSENSE
In order to maintain a constant average current while
in current limit, the LTC3766 automatically adjusts the
value of the peak current limit to cancel the effect of the
inductor ripple current. This is accomplished by creating
an internal ramp that mimics the inductor current ripple.
The amplitude of this ramp is determined by the resistor
on the IPK pin, which must be set to be proportional to
the output inductor. The LTC3766 establishes a voltage
on the IPK pin of (VSW – VS+)/15, which is one-fifteenth
of the voltage across the output inductor during the on-
time when SW is high. Therefore, it is imperative that the
SW and VS+ pins be connected as shown in Figure 14
or Figure 15 so that the LTC3766 can properly sense the
inductor voltage. If the differential amplifier is not needed,
tie VS– and VS+ together to VOUT as shown in Figure 14b.
For high VOUT applications where the SW node plateau
voltage is greater than 40V, it is necessary to add a resis-
tor divider on both the SW and VS+ pins, as shown in
Figure15. This divider will limit the voltage at the SW pin
and also impact the SG reverse overcurrent trip threshold.
See Setting the SG Reverse Overcurrent for details on
selecting the resistor divider on the SW pin.
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