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TEA1761T
Synchronous rectification controller with integrated feedback and protection
General descriptionThe TEA1761T is a member of the new generation of Synchronous Rectifier (SR)
controller ICs for switched mode power supplies. Its high level of integration allows the
design of a cost-effective power supply with a very low number of external components.
The TEA1761Tisa controllerIC dedicatedfor synchronous rectificationon the secondary
side of discontinuous conduction mode and quasi resonant flyback converters. Besides
electronics for synchronous rectification, it also has integrated circuitry for output voltage
and output current regulation.
The TEA1761T is fabricated in a Silicon On Insulator (SOI) process. This NXP SOI
process makes possible a wide range of operation.
Features
2.1 Distinctive features Combined synchronous rectification and primary feedback control functionality Wide supply voltage range (8.6 V to38V) High level of integration, resulting in a very low external component count Wide opto coupler output voltage range (3.5 V to 38V) Accurate internal voltage reference for voltage control (within 1 %) High driver output voltage of 10 V to drive all MOSFET brands to the lowest RDSon
2.2 Green features Low current consumption High system efficiency from no load to full load
2.3 Protection features Undervoltage protection Internal over-temperature protection
ApplicationsThe TEA1761T is intended for adapters. The device can also be used in all other
discontinuous conduction mode and quasi resonant flyback systems that demanda highly
efficient and cost-effective solution.
TEA1761T
GreenChip synchronous rectifier controller
Rev. 02 — 25 April 2007 Product data sheet
NXP Semiconductors TEA1761T
GreenChip synchronous rectifier controller Ordering information Block diagram
Table 1. Ordering informationTEA1761T/N2 SO8 plastic small outline package; 8 leads; body width 3.9 mm SOT96-1
NXP Semiconductors TEA1761T
GreenChip synchronous rectifier controller Pinning information
6.1 Pinning
6.2 Pin description Functional descriptionThe TEA1761T is the controller for synchronous rectification to be used in discontinuous
conduction mode and quasi resonant flyback converters. Besides controlling the MOSFET, the TEA1761T contains the voltage reference and amplifiersto regulate and
control the output voltage and current of the power supply.
7.1 Start-up and undervoltage lock outThe IC leaves the under-voltage lock-out state and activates the synchronous rectifier
circuitry and also the voltage/current sense circuitryas soonas the voltageon the VCC pin
is above 8.6 V (typical). As soon as the voltage drops below 8.1 V (typical), the
under-voltage lock-out stateis re-entered and the SR driver outputis actively kept low and
also the opto driver output is disabled.
7.2 Synchronous rectificationAfter a negative voltage (−310 mV typical) is sensed on the SRSENSE pin, the driver
output voltage is made high and the external MOSFET is switched on. As soon as the
SRSENSE voltage rises to −55 mV , the driver output voltage is regulated to maintain the
−55 mV on the SRSENSE pin. As soon as the SRSENSE voltage is above −12 mV, the
Table 2. Pin descriptionSRSENSE 1 synchronous timing input
GND 2 ground
n.c. 3 not connected
DRIVER 4 driver output for SR MOSFET
OPTO 5 opto coupler driver output
VSENSE 6 sense input for voltage control
ISENSE 7 sense input for current control
VCC 8 supply voltage
NXP Semiconductors TEA1761T
GreenChip synchronous rectifier controllerdriver output is pulled to ground. After switch-on of the SR MOSFET, the input signal on
the SRSENSE pin is blanked for 2 μs (typical). This will eliminate false switch-off due to
high frequency ringing at the start of the secondary stroke.
Because the driver output voltageis reducedas soonas the voltageon the SRSENSE pin −55 mV, the external power switch canbe switchedoff fast when the current through the
switch reaches zero. With this zero-current switch off, no separate standby mode is
neededto maintain high efficiency during no-load operation. The zero currentis detected
by sensing a −12 mV level on the SRSENSE pin. See Figure3.
If the secondary stroke of the flyback converter is shorter than 2 μs (typical), the driver
outputis disabled. This will guarantee stable operationfor very low duty cycles. When the
secondary stroke increases above 2.2 μs (typical) then the driver output is again enabled.
7.3 SMPS output voltage and current regulationThe output voltageof the flyback Switched Mode Power Supply (SMPS) canbe controlled sensing the output voltage via pin VSENSE. The feedback loop via the primary controller
can regulate the output voltage of the switched mode power supply by regulating the
voltage on pin VSENSE, to 2.5V.
Also the output current of the flyback SMPS can be controlled or limited. The voltage on
pin ISENSE is regulated or limited to 50 mV above the voltage on pin GND.
NXP Semiconductors TEA1761T
GreenChip synchronous rectifier controller
7.4 Opto outputThe opto output is intended to drive an opto coupler (see Figure 5). The opto output has open-drain output configuration. The maximum sink currentis internally limitedto5 mA
(typical). The output is linearly controlled via the VSENSE and ISENSE input pins. An
over-temperature situation will switch the opto output to its maximum sink current.
During start-up (VCC
7.5 Supply management
All (internal) reference voltages are derived from a temperature compensated, on-chip
band gap circuit. The reference voltage is trimmed to an accuracy within 1%.
7.6 OverTemperature Protection (OTP)
The IC provides an accurate internal overtemperature protection of 150 °C (typical). The
IC will maximize the current of pin OPTO as soon as the internal temperature limit is
reached. The opto signal can be used on the primary side of the flyback controller to
activate the SMPS protection or limit the output power. As soon as the overtemperature
condition is solved, normal operation will resume.
7.7 Driver
The driver circuit to the gate of the external power MOSFET has a source capability of
typically 250 mA and a sink capability of typically 2.7 A. This permits fast turn-on and
turn-off of the power MOSFET for efficient operation. The output voltage of the driver is
limited to 10 V (typical). This high output voltage will drive all MOSFET brands to the
minimum on-state resistance.
During start-up conditions (VCC voltage is actively pulled low.
NXP Semiconductors TEA1761T
GreenChip synchronous rectifier controller Limiting values
[1] Equivalent to discharging a 100 pF capacitor through a 1.5 kΩ series resistor.
[2] Equivalent to discharging a 200 pF capacitor through a 0.75 μH coil and a 10 Ω resistor. Thermal characteristics
The graphin Figure4 shows the relationship between junction temperature and VSENSE
voltage.
Table 3. Limiting values
In accordance with the Absolute Maximum Rating System (IEC 60134). All voltages are measured with respect to ground
(pin2); positive currents flow intothe chip. The voltage ratings are valid provided other ratings arenot violated; current ratings
are valid provided the other ratings are not violated.
Voltages
VCC supply voltage continuous −0.4 +38 V
VOPTO voltage on pin OPTO continuous −0.4 +38 V
VSRSENSE voltage on pin SRSENSE continuous - 120 V
VVSENSE voltage on pin VSENSE continuous −0.4 +5 V
VISENSE voltage on pin ISENSE −0.4 +5 V
Currents
IOPTO current on pin OPTO - 12 mA
IDRIVER current on pin DRIVER duty cycle<10% −0.8 +3 A
ISRSENSE current on pin SRSENSE −3- mA
General
Ptot total power dissipation Tamb <80°C - 0.45 W
Tstg storage temperature −55 +150 °C junction temperature −20 +150 °C
ESD
VESD electrostatic discharge voltage class2
human body model [1] - 2000 V
machine model [2] - 200 V
charged device model - 500 V
Table 4. Thermal characteristics
Rth(j-a) thermal resistance from junction to ambient in free air 150 K/W
NXP Semiconductors TEA1761T
GreenChip synchronous rectifier controller
10. Characteristics
Table 5. Characteristics
Tamb =25 °C; VCC=20 V; all voltages are measured with respect to ground (pin 2); currents are positive when flowing into
the IC; unless otherwise specified.
Supply voltage management (pin VCC)
Vstartup start-up voltage 8.35 8.6 8.85 V
Vhys hysteresis voltage [1] 0.5 V
ICC(oper) operating supply current VCC=8 V (VCC under normal operation; no load on
pin DRIVER 1.4 - mA
Synchronous rectification sense input (pin SRSENSE)
Vact(drv) driver activation voltage −340 −310 −280 mV
Vreg(drv) driver regulation voltage −65 −55 −45 mV
Vdeact(drv) driver deactivation
voltage
−12 mV
td(act)(drv) driver activation delay
time 125 - ns
tact(sr)(min) minimum synchronous
rectification active time
Short time 1.5 2 2.5 μs
Long time 1.7 2.2 2.7 μs
Driver (pin DRIVER)
Isource source current VCC=15 V; voltage on pin DRIVER=2V −0.3 −0.25 −0.2 A
Isink sink current VCC =15V;
voltage on pin DRIVER=2V 1 1.4 - A
voltage on pin DRIVER= 9.5V 2.2 2.7 - A
Vo(max) maximum output voltage VCC =15V - 10 12 V
