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參數(shù)資料
| 型號： | SC4910BITSTRT |
| 廠商： | Semtech Corporation |
| 英文描述： | High Performance Secondary Side Controller with Synchronous Rectifier |
| 中文描述： | 高性能二次側(cè)同步整流控制器 |
| 文件頁數(shù)： | 9/19頁 |
| 文件大小： | 330K |
| 代理商： | SC4910BITSTRT |
9
2005 Semtech Corp.
www.semtech.com
SC4910A/B
POWER MANAGEMENT
Applications Information (Cont.)
Programmable Delay
SC4910 is for single ended topologies with secondary
side synchronous rectification. It provides outputs to drive
the primary MOSFET through a small pulse transformer
and the secondary synchronous rectifiers directly. To avoid
cross conduction and optimize performance, adjustable
delay is necessary between forwarding and freewheeling
switches. The delay from falling edge of OUTB to rising
edge of OUTA is determined by a resistor from the DELAY
pin to ground. The following formula is used to calculate
the delay time:
nS
20
12
1
R
t
DELAY
+
Ε
=
where, R is the delay time setting resistor.
R should be between 20K and 200K.
The delay time from falling edge of OUTA to rising edge of
OUTB is adaptive and is triggered when the PHASE node
falls below 1.5V. If after 220nS the PHASE node has still
not fallen, the device will automatically switch.
Operation Mode
SC4910 could be configured either current mode or
voltage mode operation. In current mode, the current
sense signal comes to the CS pin while an external resistor
could configure slope compensation. In voltage mode,
an external resistor forms sawtooth with the internal 20K
resistor for voltage mode operation while current limit
signal comes to the same pin.
In current mode, which is preferred for application of
SC4910, current is sensed by a current transformer for
current feedback and over current protection. The current
in the primary switch is sensed and controlled by
developing a voltage proportional to current across a
sense resistor on the secondary. The sensed voltage is
then fed into the CS pin of SC4910. The typical current
limit threshold in the current sense pin of the SC4910 is
1.0V. The over current limit is assumed typical 120% of
full load current. Then the current sense resistor can be
calculated by the following equation:
)
pk
(
I
%
120
N
n
0
Rs
O
S
=
where
n – Power transformer primary to secondary turns ratio
N
– Secondary turns of current sense transformer
Io(pk) – Peak inductor current
An example of choosing a current sense resistor is given
below. Assume the converter full load current is 20A and
peak inductor current is 23A, the power transformer
primary to secondary turns ratio is 6:1 and the current
sense transformer primary to secondary turns ratio is
1:100, then,
≈
=
21
23
%
120
100
6
0
Rs
Slope Compensation
Slope compensation is needed to prevent sub-harmonic
oscillation at duty cycle higher than 50% and to
compensate the peak to average difference in peak
current mode control. The following equation
can be used
to calculate the external slope. If negative Se is obtained
by the equation
,
no slope compensation is needed.
S
S
N
O
L
IN
O
IN
IN
)
O
n
R
n
V
I
V
V
V
(
V
n
V
2
Se
≥
where
Se – External slope magnitude
Vin – Low input line voltage
Vo – Output voltage
n – Power transformer primary to secondary turns ratio
N
S
– Secondary turns of current sense transformer
I
L
- Peak-to-peak Inductor current ripple
For example, if the low input line voltage is 36V, output
voltage is 3.3V, power transformer primary to secondary
turns ratio is 6:1; the peak-to-peak inductor current ripple
is 6A, and current sense gain R
S
is 21W, then the external
slope needed is:
mV
85
100
6
21
6
3
6
36
)
6
3
36
(
36
6
3
2
Se
≈
≥
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