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參數(shù)資料
| 型號： | LTC3766EUFD#PBF |
| 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)： | 22/60頁 |
| 文件大?。?/td> | 607K |
| 代理商： | LTC3766EUFD#PBF |
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LTC3766
29
3766f
APPLICATIONS INFORMATION
The damping inductor LD does not carry the DC input
current. However, to ensure adequate attenuation during
large transients, choose an inductor whose saturation
current is at least:
ISAT LD
( ) ≥0.6
VOUTIOUT(MAX)
VIN(MIN)
Output Capacitor Selection
The selection of COUT is driven by the effective series
resistance (ESR) and the resulting output voltage ripple.
Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple
(
VOUT) is approximated by:
VOUT ≈ IL ESR+
1
8fSWCOUT
where fSW is the operating frequency, COUT is the output
capacitance and
IL is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since
IL increases with input voltage.
Current Sensing and Average Current Limit
The LTC3766 supports current sensing either with a cur-
rent sense resistor or with an isolated current transformer.
A current transformer is generally more efficient and has
the advantage of sensing current on the primary side in
isolated applications. This can be important because it
provides an additional safeguard against saturating the
main transformer during load transients. In addition, a
current transformer can generate a much larger current
sense signal than a sense resistor, resulting in a vastly
superior signal to noise ratio. This eases board layout
concerns for noise pickup and reduces jitter as well. Also,
theaccuracyofLTC3766currentlimitissignificantlybetter
in current transformer mode than in current sense mode.
Comparedtoacurrenttransformer,acurrentsenseresistor
is less expensive and somewhat simpler to apply than a
current transformer. When current sensing on the sec-
ondary side of an active clamp forward converter, direct
flux limit is required to prevent transformer saturation
and possible damage of the primary-side MOSFET.
This is because the current loop does not see the mag-
netizing current, and will not provide its own safeguard
against saturation. Note that in nonisolated applications,
however, the current sense resistor is placed in series
with the primary-side switch, so the current loop will be
monitoring magnetizing current.
When using a current sense resistor, the IS+ and IS– pins
operatedifferentiallyandthemaximumpeakcurrentthresh-
old is approximately 75mV. Normally, the current sense
resistor is placed in the source of the forward MOSFET,
as shown in Figure 11. Depending on PCB layout and the
shielding of the traces going to the IS+ and IS– pins, it is
sometimes necessary to add a small amount of filtering
as shown in Figure 11. Typically, values of RFL = 100Ω
and CFL = 200pF to 1nF will provide adequate filtering of
noise pickup without substantially affecting the current
loop response.
RFL
RSENSE
CFL
FORWARD
MOSFET
FG
LTC3766
IS
+
IS
–
3766 F11
Figure 11. Using a Current Sense Resistor
This filter is also helpful in correcting for the effect of the
ESL (parasitic inductance) of the sense resistor, which can
be important for RSENSE values less than 2mΩ. The effect
of the ESL is cancelled if the RC filter is chosen so that:
RFLCFL =
ESL
RSENSE
Since the LTC3766 implements an average current limit
architecture, choose the value of RSENSE based upon the
desired average current limit:
RSENSE =
55mV
ILIM(AVG)
Alternatively, if a current transformer is used to sense
the primary-side switch current, then the IS– pin should
be tied to VCC and the IS+ pin to the output of the current
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