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PDF SC4806 Data sheet ( Hoja de datos )
| Número de pieza | SC4806 | |
| Descripción | Multiple Function Double Ended PWM Controller | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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POWER MANAGEMENT
Description
The SC4806 is a double ended, high speed, highly
integrated PWM controller optimized for applications
requiring minimum space. The device is easily configurable
for current mode or voltage mode operation and contains
all the control circuitry required for isolated applications,
where a secondary side error amplifier is used.
Designed for simplicity, the SC4806 is fully featured and
requires only a few external components. It features a
programmable frequency up to 1MHZ, external
www.DatapShroeegt4raUm.commable soft start, pulse-by-pulse current limit and
over current protection for both voltage and current
modes, as well as a line monitoring input with hysteresis
to reduce stress on the power components. A ramp pin
allows for slope compensation to be programmed by
external resistors for current mode. This also allows for
operation in voltage mode with voltage feed forward.
A unique oscillator is utilized which allows two SC4806
to be synchronized together and work out-of-phase. This
feature minimizes the input and output ripples, and
reduces stress and size on input/output filter components.
The outputs are configured for push-pull format, dead time
between the 2 outputs is programmable depending on the
size of the timing components.
The SC4806 features a turn on threshold of 8 volts .
The device is available at a MLP-12 package.
Typical Application Circuit
SC4806
Multiple Function
Double Ended PWM Controller
Features
90 µA starting current
Pulse-by-pulse current limit for both voltage/current
modes
Programmable operating frequency up to 1MHz
Programmable external soft start
Programmable line undervoltage lockout
Programmable external slope compensation
Over current shutdown with separate pin
Dual output drive stages on push-pull configuration
Programmable mode of operation (peak current mode
or voltage mode)
External frequency synchronization
Bi-phase mode of operation
Lead free MLP-12 package,WEEE and RoHS
compliant
-40 to 105 °C operating temperature
Applications
Telecom equipment and power supplies
Networking power supplies
Industrial power supplies
Push-pull converter
Half bridge converter
Full bridge converter
Isolated VRMs
+VIN
-VIN
ON/OFF
REF
+
SYNC
4 RC
5 ILIM
6 RAMP
I sense
Vcc
SC4806
Vcc
VCC 12
OUTA 11
OUTB 10
Rsense
SC1301A
SC1301A
+
REF
FB
SC431
+Vo
-Vo
Revision: October 13, 2006
REF
FB
I sense
1
www.semtech.com
1 page  
SC4806
POWER MANAGEMENT
Pin Descriptions
LUVLO (Pin 1):
RC (Pin 4):
Line undervoltage lockout pin. An external resistive di-
vider from the Input supply will program the undervoltage
lockout level. The external divider should be referenced
to the quiet analog ground. During the LUVLO, the driver
outputs are disabled. This pin can also function as an
Enable/Disable.
www.DataSSShe(ePt4iUn.c2om):
An internal 35µA current source charges the external
capacitor connected to this pin. This pin is connected to
one of the inputs of the PWM comparator. When the volt-
age on this SS pin increases, but less than 1/3 of the
feedback voltage VFB, the pulse width of OUTA and OUTB
gradually increases to achieve soft start. As the output
voltage increases and feedback loop enters regulation,
the PWM modulator is controlled by VFB. At normal opera-
tion, the voltage at SS pin is clamped at Vref.
When the Over Current is tripped, both OUTA and OUTB
are pulled low after a typical time delay (Typ. 100ns). At
the same time, the SS cap is gradually discharged via an
equivalent 10µA internal current source. When the volt-
age on SS pin is dropped below 0.8V, a new SS cycle is
initiated while the SS cap is charged with 35µA again.
The internal thermal protection circuit monitors the die
temperature. If the temperature exceeds 175oC, the con-
troller is completely shutdown. When the temperature is
dropped below 160oC, defined by the hysteresis, the con-
troller re-starts with soft start process.
SYNC (Pin 3):
SYNC is a positive edge triggered input with a threshold
set to 1.75V. In a single controller operation, SYNC could
be grounded or connected to an external synchroniza-
tion clock within the SYNC frequency range. In Bi-Phase
operation mode SYNC pins could be connected to the
Cosc (Timing Capacitors) of the other controller. This will
force an out-of-phase operation (see Application Infor-
mation part).
The oscillator programming pin. The oscillator should be
referenced to a stable reference voltage for an accu-
rate and stable frequency. Only two components are re-
quired to program the oscillator, a resistor (tied to Vref
and RC), and a capacitor (tied to the RC and GND). The
following formula can be used for a close approximation
of the oscillator frequency.
FOSC
≅
1
ROSCCTOT
where:
CTOT = COSC + CSC4806 + CCircuit
CSC4806 ≅ 22pF
The recommended range of timing resistors is between
10kohm and 200kohm and range of timing capacitors
is between 100pF and 1000pF. Timing resistors less
than 10kohm should be avoided.
Refer to layout guidelines in Application Information sec-
tion to achieve best results.
ILim (Pin 5):
The current signal from a sense resistor is applied to
peak current and overcurrent comparators through ILM
pin.
Under normal operation condition, the comparators are
not trigged. When the current signal sensed at ILM pin
exceeds the first threshold -- pulse-by-pulse current limit,
the corresponding on-time is terminated for the remain-
der of the switching cycle. In this case, the circuit output
voltage loses regulation even though it continues to pro-
vide full load current.
When the load current continuously increases and the
sensed signal at ILM pin reaches the second threshold -
- over current limit, the controller turns off both OUTA
and OUTB. At the same time, the SS cap is discharged
with equivalent 10uA current source. When the voltage
at SS pin is below 0.5V, the controller initiates re-start.
The pins Ramp and ILM are discharged by the internal
FETs at the end of each switching cycle.
2006 Semtech Corp.
5
www.semtech.com
5 Page 
SC4806
POWER MANAGEMENT
Application Information (Cont.)
OPERATION MODE
SC4806 can be configured in either voltage mode or cur-
rent mode. In voltage mode, a ramp is externally gener-
ated by RC network. The R can be connected to Vref or
other fixed voltage source as shown below. By compar-
ing control signal to the ramp, PWM duty cycle is derived.
SC4806 Voltage mode (Non-Feed Forward)
www.DataSheet4U.com
Vref or other fixed Voltage source
R1
Ramp
C1
In current mode control, the ramp voltage is not derived
artificially from a ramp generator. It is instead provided
from a power converter inductor current by a current
sensing transformer or resistor. Thus a second, inner
control loop is formed by comparing the inductor current
ramp to control voltage from outer voltage loop. Now
the control voltage programs the inductor current via the
inner loop and no longer controls the duty cycle directly.
The current mode control corrects most of problems with
direct duty cycle control in voltage mode. The chief ad-
vantage of the methods its inherent feed-forward char-
acteristics and simplified loop dynamics. An added ben-
efits is the reduction or elimination of transformer satu-
ration problems in full-bridge or push-pull isolated con-
verters. The current mode configuration with SC4806 is
as shown below:
SC4806 100% Current mode (No Slope Compensation)
Isense
R3
ILIM
C2
Ramp
Voltage mode with feed-forward operation is imple-
mented if the R is connected to input voltage as shown
below. With this implementation, the ramp amplitude
varies directly with input voltage. If control signal to FB is
constant, the duty cycle varies inversely with input volt-
age. Thus the volt-second product, Vin*D, remains con-
stant without any control change. Open loop line regula-
tion better than direct duty cycle control as shown above.
Good dynamic response is achieved with less closed loop
gain required.
Isense
R3
ILIM
C2
The current mode control ling the peak inductor current
results in circuit instability whenever the steady state duty
cycle is greater than 0.5. An artificial slope has to be
added to avoid such problem. Power transformer mag-
netizing current riding on the reflected inductor current
acts to provide some slope compensation, but the
amount is rather variable and indeterminate. The cur-
rent mode with slope compensation is as shown below:
SC4806 Voltage mode (Feed Forward)
Vin
SC4806 Current mode (With Slope Compensation)
Vref or other fixed Voltage source
R1
Ramp
C1
Isense
R3
ILIM
C2
2006 Semtech Corp.
11
RC
Ramp
ILIM
Isense
www.semtech.com
11 Page | ||
| Páginas | Total 22 Páginas | |
| PDF Descargar | [ Datasheet SC4806.PDF ] | |
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