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參數(shù)資料
| 型號(hào)： | 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)： | 8/19頁 |
| 文件大?。?/td> | 330K |
| 代理商： | SC4910BITSTRT |
8
2005 Semtech Corp.
www.semtech.com
SC4910A/B
POWER MANAGEMENT
Applications Information
The SC4910A/B is a secondary side PWM controller work-
ing either in current mode or voltage mode mainly for
applications of forward converters with synchronous rec-
tification. While the OUTA drives the primary MOSFET
through transformer and the secondary forward recti-
fier, the OUTB drives the secondary freewheeling recti-
fier. The switching frequency and maximum duty cycle
can be programmable with two resistors. The delay time
from the falling edge of OUTA to the rising edge of OUTB
is adaptive by monitoring the phase node voltage. The
delay time from the falling edge of OUTB to the rising
edge of OUTA is determined by a programming resistor
from the DELAY pin to ground. The ISHARE pin allows for
current sharing among the parallel operating units to
make current equally distribute load. The -SENSE pin
separated from GND pin provides true output voltage
remote sense capability. Other features include soft start,
sychronization or enable/disable by user, provided 5V
reference voltage.
Oscillator
The frequency and duty cycle of the oscillator is controlled
by placing two resistors from the RT1 and RT2 pins to
ground. The resistor at RT1 controls the maximum “on”
duty cycle and the resistor at RT2 controls the “off”
portion of a cycle. When the resistor at RT1 is equal to
that at RT2, the maximum duty cycle will be approximately
50%. The following formula is used to determine the
time duration of the “on” and “off” portions:
10
20
×
×
=
RT
t
12
Current Sense and Current Limit
The CS pin has an input impedance of 20K ohms and
swings from 1.0V to 2.5V. With a 5K ohm resistor from
CS to ground, the device operates in voltage mode with
a ramp that will swing from 0.2V to 0.5V. When the 5K
resistor is connect to a voltage that is proportional to
the primary side current, the device will operate in current
mode. The cycle-by-cycle current limit is triggered when
the CS pin voltage rises above 1V. If CS exceeds 1.25V,
the faulty latch will be set and the outputs will be driven
low. The soft start capacitor is then discharged by the
internal current sink. No outputs are allowed until the
soft start capacitor is fully discharged to 0.15V. At this
point the fault latch will be reset and the SC4910 will
begin a soft start process. This results in a hiccup current
limit mode for continuous fault conditions.
SYNC/EN
The enable function looks at the SYNC/EN pin and an
internal timing capacitor. If the SYNC/EN pin is low and
the internal timing capacitor voltage is high, then the
SC4910 is disabled with OUTA, OUTB and SS pulled low.
When the SYNC/EN pin is held high, the device is enabled
and runs off of the internal oscillator. When a rising signal
is detected on the SYNC/EN pin a one-shot is triggered
and discharges the internal timing capacitor. As long as
the internal timing capacitor is below an internal reference
level, the device will synchronize with the external pulse.
If the internal timing capacitor is allowed to charge up to
the internal reference level before another SYNC pulse
is detected, the device will switch back to the internal
oscillator.
Soft Start
The SS pin is connected to the internal reference, 0.75V,
through an internal 6K ohm resistor. The SS pin is also
connected to the non-inverting input of the error amplifier.
With an external capacitor connected to this pin, the soft
start timing will be determined by this RC time constant.
During start-up, the SS pin is held low until the
undervoltage lockout threshold is reached. Once the
UVLO threshold is reach, the SS pin is released and the
device will regulate to the voltage on this pin.
Undervoltage Lockout
When the supply voltage V
is below the undervoltage
lockout threshold, both OUTA and OUTB are held low. The
SS pin and the COMP pin are also held low. Once the
undervoltage lockout threshold has been surpassed,
OUTA, OUTB, SS and COMP are released for normal
operation.
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