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PDF RT7781 Data sheet ( Hoja de datos )
| Número de pieza | RT7781 | |
| Descripción | PWM Flyback Controller | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT7781
PowerZeroTM Series PWM Flyback Controller
General Description
The RT7781 is a series of high-performance current mode
PWM controllers with Richtek proprietary PowerZeroTM
technology. The PowerZeroTM provides an excellent green
power solution, especially under light-load and no-load
conditions. The integrated high-voltage device provides
fast start-up function without external start-up resistors.
It also features brown-in and brown-out detection.
The RT7781 offers complete protection functions such as
internal Over-Temperature Protection (OTP), Over-Load
Protection (OLP) and VDD Over-Voltage Protection (OVP).
It also features Secondary Rectifier Short-circuit
Protection (SRSP) and Current Sensing pin (CS) open-
circuit protection. Due to these protections, design of
power supply unit becomes simple and reliable.
The RT7781 provides several versions, shown in the
RT7781 version table, for various applications. It is targeted
for cost-effective and compact power supply units such
as NB adaptors.
Marking Information
RT7781x
GSYMDNN
RT7781xGS : Product Number
x : G/L/A/T/C
YMDNN : Date Code
Features
PowerZeroTM Technology
Integrated 700V Start-Up Device
Brown-In and Brown-Out
Low No-Load Input Power (<50mW) (RT7781G/L/A/C)
Low No-Load Input Power (<30mW) (RT7781T)
Accurate Over Load Protection
Driver Capability : 400mA/−400mA
PRO Pin for Arbitrary External Protection
(RT7781G/L/A/T)
ACD Pin for TV Application (RT7781C)
Disable Mode for 30mW Solution (RT7781T)
Jittering Frequency
Soft Driving for Reducing EMI Noise
VDD Over Voltage Protection
Internal Over Temperature Protection
Secondary Rectifier Short Protection
CS Pin Open Protection
RoHS Compliant and Halogen Free
Applications
NB Adaptor
Switching AC/DC Adaptor with Low No-Load Input Power
TV and Monitor Applications
Open Frame Power Supply
Simplified Application Circuit
AC Mains
(90V to 265V)
OPTO
RT7781
VDD
HV
PRO
GATE
COMP
CS
GND
Copyright ©2014 Richtek Technology Corporation. All rights reserved.
DS7781-03 April 2014
OPTO
VOUT+
VOUT-
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
1 page  
Operation
HV pin Detection
The RT7781 provides a 700V high voltage pin to detect
the AC input voltage and supports startup.
Brown-In and Brown-Out
The brown-in and brown-out functions are designed in this
controller. Therefore, the components and power loss of
external brown-in and brown-out circuits can be eliminated.
When the HV voltage rises above the brown-in voltage
threshold (VTH_BI = 113V typ.) for more than the brown-in
deglitch time, the IC enters brown-in condition and starts
PWM switching. When the HV voltage keeps under the
brown-out voltage threshold (VTH_BO = 96V typ.) for more
than the brown-out debounce time (tD_BO), the IC enters
brown-out condition and stops PWM switching.
Burst Switching Green Mode
The burst mode is designed to reduce switching loss at
light load condition. When the output load gets light, the
COMP voltage drops and reaches VBURL, the controller
will cease switching. After the output voltage drops and
the COMP voltage goes up to VBURH, the controller will
resume switching.
VDD Holdup Mode
The RT7781 provides a unique operation mode at almost
no load condition named VDD holdup mode. Under the
VDD holdup mode, the RT7781 forces PWM switching to
maintain VDD voltage between VDD_ET and VDD_ED. The
benefit of the VDD holdup mode is to avoid the VDD drops
to VTH_OFF due to the long burst mode period at no load or
load transient moment. Therefore, this function makes bias
winging design and transient design easier and compacter.
Disable Mode (T Version Only)
The RT7781T, designed to cooperate with the specific
secondary-side voltage regulator RT7205, provides an
outstanding solution with ultra low no-load input power
(<30mW). The RT7781T features the disable mode to
minimize the power consumption at very light-load or no-
load conditions.
RT7781
Oscillator
The oscillator runs at 65kHz and features frequency
jittering function. Its jittering depth is 6% with about 4ms
envelope frequency at 65kHz. It also generates slope
compensation saw-tooth, 75% maximum duty cycle pulse
and over-load protection slope.
Leading Edge Blanking (LEB)
Due to the device parasitic capacitors, an initial current
spike appears on the current sense resistor at the
beginning of the power MOSFET on-state. The spike of
the CS voltage may incorrectly trigger the peak current
comparator to turn off the power MOSFET, resulting in
running failure of the Flyback converter. Thus, the LEB
time used to mask the initial voltage spike on the CS pin,
is a necessary design for a successful PWM operation.
Gate Driver
A totem pole gate driver is designed to meet both of low
EMI and high efficiency application requirements. The driver
integrates a voltage clamping circuit to limit the max.
GATE output voltage at high VDD supply voltage
condition(VDD > 15V). An internal pull low circuit is
activated to prevent the external MOSFET from accidental
turning-on when the VDD is pretty low during a start-up
process.
Cycle-by-Cycle Current Limit & Constant Power
This is a basic but very useful function and it can be
implemented easily in current mode controller. The entry
points of current limit are different for low line and high
line AC inputs because of the effect from different peak
currents with the same propagation delay time. The
RT7781 provides a unique calibration mechanism to
reduce the variation and achieves constant output power
between 90Vac and 265Vac.
COMP Pin Open Protection
If the COMP voltage reaches the open voltage VCOMP_OP,
the controller will shut down after about tOLP and enter
auto recovery mode.
Copyright ©2014 Richtek Technology Corporation. All rights reserved.
DS7781-03 April 2014
is a registered trademark of Richtek Technology Corporation.
5
5 Page 
AC Mains
(90V to 265V)
OPTO2
OPTO1
RT7781C
VDD 6
7 HV
1 ACD
GATE 5
2 COMP
GND
4
CS 3
OPTO1
RT7781
VOUT+
VOUT-
OPTO2
Scalar
Figure 5. RT7781C Application Circuit
Copyright ©2014 Richtek Technology Corporation. All rights reserved.
DS7781-03 April 2014
is a registered trademark of Richtek Technology Corporation.
11
11 Page | ||
| Páginas | Total 23 Páginas | |
| PDF Descargar | [ Datasheet RT7781.PDF ] | |
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