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PDF SC4808A Data sheet ( Hoja de datos )
| Número de pieza | SC4808A | |
| Descripción | High Performance Dual Ended PWM Controller | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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POWER MANAGEMENT
Description
The SC4808A/B is a dual-ended, high frequency, integrated
PWM controller, optimized for isolated applications that
require minimum space. It can be configured for current
or voltage mode operation with required control circuitry
where secondary side error amplifier is used.
Some of the key features are high frequency operation of
1 MHz that allows the use of smaller components thus
saving cost and valuable board space. An internal ramp
www.DataoShneett4hUe.coCmurrent Sense pin allows Internal Slope
Compensation programmed by an external resistor. Other
features include programmable frequency up to 1MHz,
Pulse by Pulse current and Line Monitoring Input with
Hysteresis to reduce stress on the power components.
A unique oscillator is used to synchronize two SC4808’s to
work out of phase. This minimizes the input and output
ripple thus reducing noise on the output line and reducing
stress and size of input/output filter components. The dual
outputs can be configured in Push-Pull, Half Bridge and
Full Bridge format with programmable dead time between
two outputs depending on the size of the timing
components.
The SC4808 also features a turn on threshold of 12V for
SC4808A and 4.4V for SC4808B. They are available in
MSOP-10 packages.
Typical Application Circuit
SC4808A/B
High Performance Dual
Ended PWM Controller
Features
120µA starting current
Pulse by pulse current limit
Programmable operation up to 1MHz
Internal soft start
Programmable line undervoltage lockout
Over current shutdown
Dual output drive stages on push-pull configuration
Programmable internal slope compensation
Programmable mode of operation (peak current mode
or voltage mode)
External frequency synchronization
Bi-phase mode of operation
-40 to 105 °C operating temperature
10 Pin MSOP lead free package available. WEEE and
RoHS compliant.
Applications
Telecom equipment and power supplies
Networking power supplies
Industrial power supplies
Push-pull converter
Half bridge converter
Full bridge converter
Isolated VRM’s
Vo
Vin Gnd_Out
Gnd_In
RSENSE
Revision October 20, 2005
OUTA RC
OUTB REF
CS
SYNC
VCC
SYNC
FB
LUVLO GND
SC4808
1
www.semtech.com
1 page  
SC4808A/B
POWER MANAGEMENT
Pin Descriptions
FB: The inverting input to the PWM comparator. Stray in-
ductances and parasitic capacitance should be minimized
by utilizing ground planes and correct layout guide lines
(see page 22).
REF: Bandgap reference output It should be by passed with
a 2.2uF low ESR capacitance, right at the IC pin.
LUVLO: Line undervoltage lockout pin. An external resis-
tive divider will program the undervoltage lockout level. The
external divider should be referenced to the quiet analog
ground (see page 22). During the LUVLO, the driver out-
puts are disabled and the softstart is reset. This pin can
also function as an Enable/Disable.
www.DataCShSe:etC4Uu.rcroemnt sense input and internal slope compensation
are both provided via the CS pin. The current sense input
from a sense resistor is used for the peak current and
overcurrent comparators. An internal 1 to 3 feed back volt-
age divider provides a 3X amplification of the CS signal.
This is used for comparison to the external error amplifier
signal. If an external resistor is connected from CS to the
current sense resistor, the internal current source will pro-
vide a programmable slope compensation. The value of
the resistor will determine the level of compensation. At
higher compensation levels, voltage mode of operation can
be achieved.
RC: 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 follow-
ing formula can be used for a close approximation of the
oscillator frequency.
FOSC _ A
≅
1
ROSC CTOT
× 0.8
FOSC _ B
≅
1
ROSC CTOT
× 0.9
SYNC: SYNC is a positive edge triggered input with a thresh-
old set to 1.6V (SC4808A), and 1.0V (SC4808B).
In a single controller operation, SYNC could be grounded
or connected to an external synchronization clock within
the SYNC frequency range (see page 3). In the 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 page 15).
GND: Device power and analog ground. Careful attention
should be paid to the layout of the ground planes (see page
22).
OUTA and OUTB: Out of phase gate drive stages. The
driver’s peak source and sink current drive capability of
100mA, enables the use of an external MOSFET driver or
a NPN/PNP transistor buffer.
The oscillator RC network programs the oscillator frequency,
which is twice the OUTA/OUTB frequency. To insure that
the outputs do not overlap, a dead time can be generated
between the two outputs by sizing the oscillator timing ca-
pacitor (see page 14).
where:
CTOT = COSC + CSC 4808 + CCircuit
CSC 4808 ≅ 22 pF
VCC: The supply input for the device. Once VCC has ex-
ceeded the UVLO limit, the internal reference, oscillator,
drivers and logic are powered up. A low ESR capacitance,
should be used for decoupling right at the IC pin to mini-
mize noise problems.
Where the frequency is in Hertz, resistance in ohms, and
capacitance in farads. The recommended range of timing
resistors is between 10 kohm and 200kohm and range of
timing capacitors is between 100pF and 1000pF. Timing
resistors less than 10 kohm should be avoided.
Refer to layout guide lines on page 22 to achieve best re-
sults.
2005 Semtech Corp.
5
www.semtech.com
5 Page 
SC4808A/B
POWER MANAGEMENT
SC4808B Typical Characteristics (Cont.)
100
98
Vcc UVLO (Hysteresis)
96
94
92
90
www.DataShee88t4U.com
86
84
-40
-25 -10 5 20 35 50 65 80 95 110
Ta (°C)
Vcc UVLO Hysteresis vs. Temperature
125
1400
1200
Oscillator Frequency 1MHz
Oscillator Frequency 500kHz
1000
800
600
400
200
-40 -25 -10 5 20 35 50 65 80 95 110
Ta (°C)
Oscillator Frequency vs. Temperature
125
3.170
3.165
LUVLO (Rising)
3.160
3.155
3.150
3.145
3.140
3.135
3.130
3.125
3.120
-40 -25 -10 5 20 35 50 65 80 95 110 125
Ta (°C)
Line UVLO vs. Temperature
664
662
Sync. Frequency @ Fosc = 500kHz
660
658
656
654
652
650
-40 -25 -10
5
20 35 50 65 80 95 110 125
Ta (°C)
Synchronization Frequency vs. Temperature
140
135
LUVLO (Hysteresis)
130
125
120
115
110
105
100
-40 -25 -10
5
20 35 50 65 80 95 110
Ta (°C)
Line UVLO Hysteresis vs. Temperature
125
49.5
49.5
Maximum Duty Cycle
49.5
49.4
49.4
49.4
49.4
49.4
-40 -25 -10
5
20 35 50 65 80 95 110 125
Ta (°C)
Maximum Duty Cycle vs. Temperature
2005 Semtech Corp.
11
www.semtech.com
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet SC4808A.PDF ] | |
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