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參數(shù)資料
| 型號(hào)： | SC4901 |
| 廠商： | Semtech Corporation |
| 英文描述： | COMBI-SYNC Secondary Side Synchronous Rectifier and Regulator |
| 中文描述： | 客貨兩用，同步二次側(cè)同步整流和穩(wěn)壓器 |
| 文件頁數(shù)： | 9/20頁 |
| 文件大?。?/td> | 598K |
| 代理商： | SC4901 |
9
2005 Semtech Corp.
www.semtech.com
SC4901
POWER MANAGEMENT
Application Information (Cont.)
The RCT pin has a timing capacitor CT connected to AGND
and a charging resistor RT connected to the source, which
is typically derived from the transformer voltage. The RCT
pin is clamped to 3V peak internally and the ramp operates
between 0V and 3V. The high side forward pulse is
terminated whenever the RCT ramp hits the 3V peak.
Ensure that the ramp reaches this peak only after the
negative transition of the transformer voltage, even under
the worst case conditions of high line and maximum
applicable duty ratio. Otherwise the available pulse width
for post regulation will be limited. On the other hand, the
timing capacitor should get charged fully to 3V prior to
positive transition of the transformer voltage. This will
ensure that the dead time between the rectifying and
forward gate drives is consistently maintained. Ideally the
ramp should be charged to its peak of 3V just after the
falling edge of the transformer secondary voltage so that
the voltage error amplifier output utilises the full range of
0 to 3V. This is shown in the idealised waveforms below.
Fig 2. Idealised ZCD and RCT waveforms
The timings for the idealised waveforms above are easier
to implement if the transformer has some form of constant
volt-second operation or control; the secondary voltage can
be directly used to charge the timing capacitor CT. The
maximum input current into the RCT pin under clamped
conditions should be limited to 2 mA.
Turn On Sequence
Prior to transformer voltage going positive, the ramp
capacitor is at its maximum of 3.0V. On the rising edge of
transformer secondary, 0.75V at the ZCD pin is detected
first and OUTB gate drive goes low which turns off the low
side rectifying FET QR (Fig. 1). Simultaneously a current
sink of 10 mA is activated to discharge the timing capacitor
CT to the lower threshold of 1V. This discharge time
provides the delay between turn off of rectifying FET and
turn on of the forward MOSFETs and is given by
2
×
=
CT
DLon
where CT is the ramp capacitor value in pF and DLon is
the dead time in nS. The actual dead time is extended by
the propagation delays internal to the controller, which
should be taken into account while choosing CT. The
propagation delays are typically in the range of a few tens
of nanoseconds.
When the RCT pin goes down to 1V, OUTA is enabled to
drive the high side pair of FETs. Meanwhile, on the rising
edge of ZCD pin, a higher threshold of 3.5V is detected.
Once both these conditions have been met, OUTA goes
high, XFRA and the forward FETs QS and QF are turned on.
Setting the RCT Ramp
The CT capacitor is typically charged from the transformer
secondary voltage through RT. The current through RT is
given by (V
SEC
- V
FWD
- V
CT
) / RT where
V
SEC
V
FWD
V
CT
= Peak of the transformer secondary voltage
= Forward drop of the signal diode (D1 in Fig 1)
= Instantaneous capacitor voltage
As a first approximation the average current can be
assumed to be (V
SEC
- 2V ) / RT.
ZCD INPUT
XFR SEC
VOLTAGE
RCT RAMP
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相關(guān)代理商/技術(shù)參數(shù) |
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| SC4901ITSTRT | 制造商:Semtech Corporation 功能描述:Secondary Side PWM Cntrlr SYNC POST REGUL 1MHz 16-Pin TSSOP T/R 制造商:Semtech Corporation 功能描述:Controller IC |
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| SC4905A_06 | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Voltage Mode PWM Controller |
| SC4905AIMSTR | 制造商:Semtech Corporation 功能描述:IC, VOLTAGE MODE PWM CTRL, 12V, MSOP-10; Input Voltage:48V; Output Voltage:500mV; Output Current:1mA; Supply Voltage Min:0V; Supply Voltage Max:12V; Digital IC Case Style:MSOP; No. of Pins:10; Frequency:1MHz; Control Mode:Voltage ;RoHS Compliant: No |
| SC4905AIMSTRT | 制造商:Semtech Corporation 功能描述:Voltage Mode PWM Controller 10-Pin MSOP T/R |
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