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STRX6750 Datasheet Preview

STRX6750 Datasheet

Off-Line Quasi-Resonant Switching Regulators

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Product Information
STR-X6700 Series Off-Line
Quasi-Resonant Switching Regulators
Introduction
The STR-X6700 series integrates a quasi-resonant control
IC and a MOSFET with avalanche guarantee. In normal
operation, the device provides high efficiency and low EMI
noise with bottom-skip quasi-resonant operation during
light output loads. Low power consumption is also achieved
by Auto-standby mode (not available in the STR-X6729,
STR-X6759N, or STR-X6768N) or manual standby mode
(external trigger).
The device is supplied in a seven-pin fully-molded TO-3P-
style package, which is suitable for downsizing and stan-
dardizing of an SMPS by reducing external component
count and simplifying circuit design.
Features and benefits include the following:
Auto standby mode (burst oscillation) or manual standby
mode (UVLO intermittent oscillation) in the standby
mode.
In addition to the standard quasi-resonant operation, a
bottom-skip mode is available for increased efficiency
from light to medium load.
Soft-start operation at start-up.
Reduced switching noise (compared to conventional
PWM hard-switching solution) with a step-drive
function.
Built-in avalanche-energy-guaranteed power MOSFET
(to simplify surge-absorption circuit; no VDSS derating is
required).
Overcurrent protection (OCP), overvoltage protection
(OVP), overload protection (OLP), and maximum
On-time control circuits are incorporated; OVP and OLP
go into a latched mode.
The product lineup for the STR-X6700 series provides the
options shown in table 1.
Contents
Introduction
Pin functional descriptions
Operation description
Transformer parameters
General considerations
Design considerations
Package dimensions, TO-3P
Worldwide Contacts
1
2
6
10
11
13
14
19
LF1901
LF1902
LF1905
Figure 1. STR-X6700 series packages are fully molded TO-3P
package types: LF1901 (STR-X6737 and STR-X6769), LF1902
(STR-X6729 and STR-X6768N), and LF1905 (STR-X6757 and
STR-X6759N).
Table 1. Product Line-up
Type #
STR-X6729b
MOSFET
VDSS
(V)
450
RDS(on)
(Max)
(Ω)
0.189
VAC
Input
(V)
120
POUTa
(W)
460
STR-X6737 500 0.36 120
280
STR-X6757
STR-X6759Nb
650
0.62
0.385
Wide
230
Wide
230
165
320
250
460
STR-X6768Nb
STR-X6769
800
Wide
1.00
230
Wide
0.66
230
150
220
210
310
aThe listed output power represents thermal ratings, and the peak
output power, POUT , is obtained by 120% to 140% of the thermal
rating value. In case of low output voltage and narrow on-duty
cycle, the POUT (W) becomes lower than the above.
bAuto-standby mode not included.
All performance characteristics given are typical values for
circuit or system baseline design only and are at the nominal
operating voltage and an ambient temperature of 25°C, un-
less otherwise stated.
28103.3013




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STRX6750 Datasheet

Off-Line Quasi-Resonant Switching Regulators

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Pin functional descriptions
VCC Supply (pin 4)
Start-up circuit The start-up circuit detects the VCC pin volt-
age, and makes the control IC start and stop operation. The power
supply of the control IC (VCC pin input) employs a circuit as
shown in figure 2. At start-up, C3 is charged through a start-up
resistor R2. The R2 value needs to be set for more than the hold
current of the latch circuit (140 μA max.) and to operate at the
minimum AC input.
If the value of R2 is too high, the C3 charge current will be
reduced. Consequently, it will take longer to reach the Operation-
Start voltage. The VCC pin voltage falls immediately after
the control circuit starts its operation. The voltage drop can be
reduced by increasing C3 capacitance. However, too large a
C3 capacitance will cause an improperly long time to reach the
Operation-Start voltage after the initial power turn on.
In general, SMPS performs its start-up operation properly
with a value of C3 between 4.7 and 47 μF, and R2 between
47 and 150 kΩ for 120 V narrow or universal AC input, and
82 to 330 kΩ for 200 V narrow AC input.
As shown in figure 3, the circuit current is limited to 100 μA max
(VCC = 15 V, and resistor R2 with appropriate high resistance
value for the circuit) until the control circuit starts its operation.
Once the VCC pin voltage reaches 18.2 V, the control circuit
starts its operation by the start-up circuit, and supply current is
increased. Once the VCC pin voltage drops down to lower than
the Operation-Stop voltage 9.7 V, the UVLO circuit operates to
stop the control circuit, and the IC returns to its initial state.
Bias/drive winding After the control circuit starts its operation,
the power supply is operated by rectifying and smoothing the
voltage of the bias winding. Figure 4 shows the start-up voltage
waveform of the VCC pin. The bias winding voltage does not
immediately increase up to the set voltage after the control circuit
starts its operation. That is why the VCC pin voltage starts drop-
ping. The Operation-Stop voltage is set as low as 10.6 V (max),
the bias winding voltage reaches a stabilized voltage before it
drops to the Operation-Stop voltage, and the control circuit conti-
ues its operation. The bias winding voltage, in normal power sup-
ply operation, is set for the voltage across C3 to be higher than
the Operation-Stop voltage, VCC(OFF) , 10.6 V (max.) and lower
than the OVP-operation voltage, VCC(OVP) , 25.5 V (min.).
I CC
100 μ A
( MAX)
9.7 V
( TYP)
VC C
15V 18. 2 V
( TYP)
Figure 3. VCC pin current versus voltage
R2
1
D
VCC 4
STR-X6700
GND 3
C3
P
D2
D
Figure 2. External start-up circuit.
VCC
18.2 V
( TYP)
Operation Start
Bias Winding Voltage
10.6V
(MAX)
AC on
Start-up
failure
time
Figure 4. VCC pin voltage after start-up, capacitor C3 installed
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
28103.3013


Part Number STRX6750
Description Off-Line Quasi-Resonant Switching Regulators
Maker Sanken
Total Page 19 Pages
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