Allegro STR-X6757 INSTALLATION INSTRUCTIONS
STR-X6757
Off-Line Quasi-Resonant Switching Regulators
Features and Benefits
▪ Quasi-resonant topology IC Low EMI noise and soft
switching
▪ Bottom-skip mode Improved system efficiency over
the entire output load by avoiding increase of switching
frequency
▪ Auto-Standby mode Lowers input power at very light
output load condition
▪ Avalanche-guaranteed MOSFET Improves system-level
reliability and does not require V
▪ 650 V
▪ Various protections Improved system-level reliability
▫ Pulse-by-pulse drain overcurrent limiting
▫ Overvoltage Protection (bias winding voltage sensing),
with latch
▫ Overload Protection with latch
▫ Maximum on-time limit
/ 0.62 Ω R
DSS
DS(on)
derating
DSS
Package: 7-Pin TO-3P
Description
The STR-X6757 is a quasi-resonant topology IC designed for
SMPS applications. It shows lower EMI noise characteristics
than conventional PWM solutions, especially at greater than
2 MHz. It also provides a soft-switching mode to turn on the
internal MOSFET at close to zero voltage (VDS bottom point)
by use of the resonant characteristic of primary inductance
and a resonant capacitor.
The package is a fully molded TO-3P, which contains the
controller chip (MIC) and MOSFET, enabling output power
up to 320 W at 230 VAC and 165 W at universal input. The
bottom-skip mode skips the first bottom of VDS and turns on the
MOSFET at the second bottom point, to minimize an increase
of operating frequency at light output load, improving systemlevel efficiency over the entire load range.
There are two standby modes available to reduce the input power
under very light load conditions. The first is Auto-Standby
mode, which is internally triggered by periodic sensing, and
the other is a manual standby mode, which is executed by
clamping the secondary output. In general applications, the
manual standby mode reduces the input power further compared
to Auto-Standby mode.
Not to scale
Continued on the next page…
T ypical Application
28103.30-10
STR-X6757
Off-Line Quasi-Resonant Switching Regulators
Description (continued)
The soft-start mode minimizes surge voltage and reduces power stress
to the MOSFET and to the secondary rectifying diodes during the
start-up sequence. Various protections such as overvoltage, overload,
overcurrent, maximum on-time protections and avalanche-energyguaranteed MOSFET secure good system-level reliability.
Applications include the following:
▪ Set Top Box
▪ LCD PC monitor, LCD TV
▪ Printer, Scanner
▪ SMPS power supplies
Selection Guide
Part Number Package
STR-X6757 TO-3P
Absolute Maximum Ratings at T
= 25°C
A
Parameter Symbol Conditions Rating Unit
Drain Current
Maximum Switching Current
Single Pulse Avalanche Energy
Input Voltage for Controller (MIC) V
SS/OLP Terminal Voltage V
FB Terminal Inflow Current I
FB Terminal Voltage V
OCP/BD Terminal Voltage V
MOSFET Power Dissipation
Controller (MIC) Power Dissipation P
Operating Internal Leadframe Temperature T
Operating Ambient Temperature T
Storage Temperature T
Channel Temperature T
1
Refer to MOSFET ASO curve
2
I
DMAX
3
Refer to Avalanche Energy Derating curve
4
Refer to MOSFET Ta-PD1 curve
1
2
3
4
IDpeak Single pulse 18 A
I
DmaxTA
E
AS
CC
SSOLP
FB
FBIFB
OCPBD
P
D1
D2
OP
stg
ch
= –20°C to 125°C 18 A
Single pulse, V
within the limits of I
= 30 V, L = 50 mH, I
DD
FB
Lpeak
With infinite heatsink 44 W
Without heatsink 2.8 W
V
× I
CC
CC
Recommended operation temperature, see cautions –20 to 125 °C
F
is the drain current determined by the drive voltage of the IC and the threshold voltage, Vth, of the MOSFET
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, T
, of 25°C, unless oth er wise stated.
A
= 3.53 A 326 mJ
35 V
–0.5 to 6.0 V
10 mA
–0.5 to 9.0 V
–1.5 to 5.0 V
0.8 W
–20 to 125 °C
–40 to 125 °C
150 °C
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
STR-X6757
Functional
Block Diagram
Off-Line Quasi-Resonant Switching Regulators
Terminal List Table
Number Name Description Functions
1 D Drain MOSFET drain
2 S Source MOSFET source
3 GND Ground terminal Ground
4 VCC Power supply terminal Input of power supply for control circuit
5 SS/OLP Soft Start/Overload Protection terminal Input to set delay for Overload Protection and Soft Start operation
6 FB Feedback terminal
7 OCP/BD Overcurrent Protection/Bottom Detection Input for Overcurrent Detection and Bottom Detection signals
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Input for Constant Voltage Control and Burst (intermittent) Mode
oscillation control signals
3
STR-X6757
0
20
40
60
80
100
0 20 40 60 80 100 120
A.S.O. temperature derating coefficient (%)
TF [℃]
Internal frame temperature
STR-X6757
A.S.O. tem perature derating coefficient
curve
0
20
40
60
80
100
25 50 75 100 125 150
E
AS
temperature derating coefficient (%)
Tch [℃]
Channel temperature
STR-X6757
Avalanch e energy d erating cu rve
0.01
0.1
1
10
100
1 10 100 1000
I
D
[A]
Drain Current
VDS[V]
Drain-to-Source Voltage
STR-X6757
MOSFET A.S.O. Curve
Determined by On-Resistance
1ms
100μs
Off-Line Quasi-Resonant Switching Regulators
App ly A.S.O temp erature
co efficient fr om the l eft graph to
th is cur ve fo r ev aluating actual
op erations' safety
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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