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參數(shù)資料
| 型號： | SC4901ITSTRT |
| 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)： | 10/20頁 |
| 文件大?。?/td> | 598K |
| 代理商： | SC4901ITSTRT |
10
2005 Semtech Corp.
www.semtech.com
SC4901
POWER MANAGEMENT
Application Information (Cont.)
Having chosen CT to meet dead time requirements, the
resistor is chosen so that the ramp voltage is just below
3.0V peak at the end of the maximum ON time.
where
RT
Fsw
D
CT
= Timing resistor in k
= Switching frequency in kHz
= Operating duty cycle at which Vsec is applied
= Timing capacitor in pF
If the primary side is PWM modulated in a feedback loop
or has a volt second clamp, the product Vsec x D is fairly
constant. If it is free running with a fixed duty cycle, use
the maximum value of Vsec. The formula assumes that
Vsec is much larger than the ramp voltage of 3.0V. For
lower secondary voltages, use a smaller value of RT.
REF Output and Error Amplifier
The reference level for output voltage feedback is 0.75V
bandgap. This is amplified, buffered and put out as a 2.5V
REF output. The REF voltage can be used for external
monitoring circuits or for regulating output voltages less
than 0.75V. The output of the error amplifier can swing
between 0.3V to 2.9V, just below the clamped peak of the
timing ramp. The REF pin should be bypassed to AGND
with a 0.1 uF ceramic capacitor. A maximum of 5 mA can
be drawn from the REF output.
Delays and Turn Off Sequence
The delay between turn off of the rectifying MOSFET QR to
the turn on of the forward MOSFETs QS and QF is
determined by the discharge of the ramp capacitor CT
through a 10 mA current. The other delay, between turn
off of the forward MOSFETs and turn on of the rectifying
MOSFET is fixed and typically 110 nS. This delay is valid
when the output is in regulation and the forward FETs are
turned off by the PWM comparator. During transients or if
the output loses regulation, QS and QF will be turned off
on the falling edge of the transformer voltage. First, the
high comparator detects the 2.5V threshold and pulls down
OUTA. This turns off the forward FETs. Further down, the
lower comparator detects the 0.25V threshold and turns
on OUTB. This ensures that the forward FET pair is off
before transformer voltage goes negative.
Current Sense and Current Limit
Current sensing is done as the input of an uncommitted
differential amplifier. The current limit threshold is 2.5V at
the CAO pin. This allows the user considerable flexibility to
design the current sensing circuit. A very low value sense
resistor which results in a current signal of a few tens of
millivolts or inductor drop sensing can be used for simple
protection and maximum efficiency. The current amplifier
gain has to be correspondingly higher. A larger current
sense signal with lower gain may be preferred for better
noise immunity and more precise current limit. Maximum
recommended differential input is 200 mV and minimum
differential gain for the current amplifier is 10, to ensure
proper operation. The CAO output is typically 2V at full load.
This signal can be brought out and used further for current
sharing applications. The current sense pins have a
maximum common mode range of AVCC - 2V.
When 2.5V limit is reached at CAO pin, the bidirectional
forward switch QS/QF is turned off for the rest of the cycle.
The rectifying FET is kept ON throughout to discharge the
high inductor current. As the overload level is increased,
the ON time continues to reduce and the output drops
gradually. The output is continuously monitored for
undervoltage and when it falls below 50% of the nominal
voltage, an abnormal condition is detected and both the
outputs are shut down. The soft start capacitor is
discharged by a 2 uA current sink down to 0.8V at which
point a soft start cycle is initiated to restart the convertor.
This effectively provides hiccup mode of protection under
short circuit.
Output Drivers
There are two complementary outputs designated A and
B. Output A and XFRA are the PWM drives that control the
bidirectional forward MOSFETs. The complementary output
for driving the ground referenced rectifying MOSFET QR is
designated as OUTB. This drive is capable of sourcing and
sinking +1A. OUTA is enabled when the transformer
voltage turns on and ZCD voltage exceeds 3.5V. It is turned
off on any one of the three conditions.
a) the ramp voltage on RCT exceeds the error amplifier
output at VEA or
b) ZCD detects that the transformer voltage is turning off
and gone below 2.5V or
c) overcurrent is detected at 2.5V on the CAO pin.
CT
Fsw
D
2)
(Vsec
300000
RT
×
×
×
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