CX7508

High Precision CC/CV Primary-Side PWM Power Switch

GENERAL DESCRIPTION FEATURES

■ ± 5% Constant Voltage Regulation and
CX7508 is a high performance offline PWM power Current Regulation at Universal AC Input
switch for low power AC/DC charger and adapter ■ Primary Side Regulation (PSR) Without TL431
applications.It operates in primary-side sensing and Opto-coupler
and regulation.Consequently, opto-coupler and ■ No Need for Control Loop Compensation
TL431 could be eliminated. High Precision ■ Programmable CV and CC Regulation
Constant Voltage (CV) and Constant Current (CC) ■ Multi-Mode PWM/PFM Operation
control are integrated as shown in the figure below. ■ Output Short Load Protection
In CC control, the current and output power setting ■ Less than 75mW Standby Power
can be adjusted externally by the sense resistor Rs ■ Built-in Line Voltage and Primary Winding
at CS pin. In CV control, multi-mode operations are Inductance Compensation
utilized to achieve high performance and high ■ Built-in Adaptive Current Peak Regulation
efficiency. In addition, good load regulation is ■ Programmable Cable Drop Compensation
achieved by the built-in Cable Drop Compensation. ■ Audio Noise Free Operation
Device operates in PFM in CC mode as well at ■ Built-in Over Temperature Protection (OTP)
large load condition and it operates in PWM with ■ Soft Gate Driver for Good EMI Performance
frequency reduction at light/medium load. The chip ■ Built-in Leading Edge Blanking (LEB)
consumes very low operation current, it can ■ Cycle-by-Cycle Current Limiting
achieve less than 75mW standby power. ■ VDD Under Voltage Lockout with Hysteresis
CX7508 offers comprehensive protection coverage (UVLO)
with auto-recovery features including ■ VDD OVP & Clamp
Cycle-by-Cycle current limiting, VDD Over Voltage
Protection (VDD OVP), short circuit protection,
built-in leading edge blanking, VDD under voltage APPLICATIONS
lockout (UVLO), OTP, etc. Low Power AC/DC offline SMPS for
High precision constant voltage (CV) and constant ■ Cell Phone Charger
current (CC) can be achieved by CX7508. ■ Digital Camera Charger
CX7508 is offered in DIP7 package. ■ Small Power Adapter
■ Auxiliary Power for PC, TV etc.
■ Linear Regulator/RCC Replacement

TYPICAL APPLICATION

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

GENERAL INFORMATION
Pin Configuration Package Dissipation Rating
The pin map of DIP7 package is shown as Package RJA (℃/W)
below. DIP7 75

Absolute Maximum Ratings

Parameter Value
Drain Voltage (off state) -0.3 to 650V
VDD Zener Clamp Voltage VDD_Clamp
VDD Clamp Continuous
10 mA
Current
CS Input Voltage -0.3 to 7V
INV Input Voltage -0.3 to 7V
Maximum Operating Junction
150 oC
Temperature TJ
Min/Max Storage Temperature
-55 to 150 oC
Tstg
Lead Temperature (Soldering,
260 oC
10secs)
Note: Stresses beyond those listed under “absolute maximum
ratings” may cause permanent damage to the device. These
are stress ratings only, functional operation of the device at
these or any other conditions beyond those indicated under
“recommended operating conditions” is not implied. Exposure
to absolute maximum-rated conditions for extended periods
Ordering Information may affect device reliability.
Part Number Description

DIP7, Rohs, 50Pcs/Tube

CX7508

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

TERMINAL ASSIGNMENTS

Pin Num Pin Name I/O Description

1 VDD P Power supply.
2 NC - No connect.
The voltage feedback from auxiliary winding. Connected to resistor divider
3 INV I
from auxiliary winding reflecting output voltage.
4 CS I Current sense input.
5/6 Drain O The Power MOSFET Drain
7 GND P Ground.

BLOCK DIAGRAM

RECOMMENDED OPERATING CONDITION

Symbol Parameter Min Max Unit

VDD VDD Supply Voltage 11 27 V
TA Operating Ambient Temperature -40 85 o
C

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

ELECTRICAL CHARACTERISTICS
(TA = 25OC, VDD=18V if not otherwise noted)
Symbol Parameter Test Conditions Min Typ Max Unit
Supply Voltage Section (VDD Pin)
Start-up current into VDD
IVDD_st 2 20 uA
pin
IVDD_Op Operation Current VINV=3V, VDD=20V 1 1.5 mA
IVDD_standby Standby Current 0.5 1 mA
VDD Under Voltage
VDD_ON 15 16.3 17.5 V
Lockout Exit
VDD Under Voltage
VDD_OFF 8 9 10 V
Lockout Enter
VDD_OVP VDD OVP Threshold 28 30 32 V
VDD Zener Clamp
VDD_Clamp I(VDD ) = 7 mA 32.5 34.5 36.5 V
Voltage
Feedback Input Section (INV Pin)
Internal Error Amplifier
VINV_REF 1.97 2.0 2.03 V
(EA) Reference Input
Short Load Protection
VINV_SLP 0.7 V
(SLP) Threshold
Short Load Protection
TINV_Short 10 ms
(SLP) Debounce Time
Demagnetization
VINV_DEM 25 mV
Comparator Threshold
Toff_min Minimum OFF time 2 us
Toff_max Maximum OFF time 5 ms
Maximum Cable Drop
ICable_max 63 uA
compensation current
Current Sense Input Section (CS Pin)
CS Input Leading Edge
TLEB 500 ns
Blanking Time
Vcs(max) Current limiting threshold 490 500 510 mV
Over Current Detection
TD_OCP 100 ns
and Control Delay
Over temperature Protection
TSD Thermal Shutdown --- 165 -- °C

TRC Thermal Recovery 135 -- °C

Power MOSFET Section (Drain Pin)

Power MOSFET Drain
VBR Source Breakdown 650 V
Voltage
Static Drain-Source On
Rdson 1.4 Ohm
Resistance

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

CHARACTERIZATION PLOTS

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

OPERATION DESCRIPTION
CX7508 is a cost effective PWM power switch between the auxiliary winding and INV (pin 3),
optimized for off-line low power AC/DC the auxiliary voltage is sampled at the end of the
applications including battery chargers and demagnetization and it is hold until the next
adapters. It operates in Primary Side Regulation sampling. The sampled voltage is compared with
(PSR), thus opto-coupler and TL431 are not VINV_REF (2.0V) and the error is amplified by an
required. High precision CV and CC control can internal Error Amplifier (EA). The internal EA
be achieved to meet most small power charger output reflects the load condition and control the
and adapter applications requirements. PWM switching frequency to regulate the output
voltage, thus constant voltage (CV) can be
 Startup Current and Start up Control achieved.
Startup current of CX7508 is designed to be very When system enters over load condition, the
low (typically 2uA) so that VDD could be charged output voltage falls down and the INV sampled
up above UVLO threshold level and device starts voltage should be lower than 2V internal
up quickly. A large value startup resistor can reference which makes system enter CC Mode
therefore be used to minimize the power loss yet automatically.
reliable startup in application.
 Multi Mode CV Operation for High
 Operating Current Efficiency
The operating current of CX7508 is as low as
CX7508 is a multi-mode PSR controller. In CV
1mA. Good efficiency is achieved by the low
mode, the controller changes the mode of
operating current together with extended burst
operation by sampling INV voltage. Under the
mode control schemes at No/light
medium to large load conditions, the IC operates
load conditions. in PWM mode, which improves the system audio
noise performance. Under normal to light load
 Primary Side CC/CV Control
conditions, the IC operates in PFM mode to
CX7508 is designed to produce good CC/CV
achieve high efficiency. The IC can achieve less
control. In charger applications, a discharged
than 75mW standby power, as shown in Fig.2
battery charging starts in CC portion of the curve
until it is nearly full charged and smoothly
switched to operate in CV portion of the curve. In
an AC/DC adapter, the normal operation occurs
only on the CV portion of the curve. The CC
portion provides output current limiting.

Fig.2

 CC Operation Switching Frequency

The switching frequency of CX7508 is adaptively
controlled according to the load conditions and
the operation modes.
For flyback operation in DCM, the maximum
output power is given by
Fig.1 1
Po MAX  L P FSW I P2
As shown by Fig 1, when the PWM switching is 2
off the secondary side diode is conducted and Where Lp indicate the inductance of primary
the auxiliary winding voltage is proportion to the winding and Ip is the peak current of primary
output voltage. Via a resistor divider connected winding.

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

Refer to the equation above, the change of the response performance, which enables charger
primary winding inductance results in the change system to meet the USB charge requirements.
of the maximum output power and the constant
output current in CC mode. To compensate the  Programmable Cable Drop Compensation
change from variations of primary winding
inductance, the switching frequency is locked by In CX7508 , cable drop compensation is
and internal loop such that the switching implemented to achieve good load regulation. An
frequency is offset voltage is generated at INV pin by an
1 internal current flowing into the resistor divider.
FSW  The current is proportional to the switching off
2TDEM time, as a result, it is inversely proportional to the
Since TDEM is inversely proportional to the output load current, thus the drop due to the
inductance, as a result, the product of Lp and cable loss can be compensated. As the load
Fsw is constant, thus the maximum output power current decreases from full-load to no-load, the
and constant current in CC mode will not change offset voltage at INV pin will increase. It can also
as primary winding inductance changes. Up to be programmed by adjusting the resistance of
± 10% variation of the primary winding the divider to compensate the drop for various
inductance can be compensated. cable lines used.
The percentage of maximum compensation is
 Adjustable CC Point and Output Power V Icable_max  R1//R2 
In CX7508, the CC point and maximum output   100%
power can be externally adjusted by external Vout VINV_REF
current sense resistor Rs at CS pin as illustrated Δ V is load compensation voltage and Vout is
in typical application diagram. The larger Rs, the output voltage;
smaller CC point is, and the smaller output For example: R1=3K Ω , R2=18K Ω , The
power becomes, and vice versa as shown in percentage of maximum compensation is given
Fig.3. by
V 63uA  3K//18K 
  100%  8.1%
Vout 2V

 Over Temperature Protection (OTP)

When the IC temperature is over 165 OC, the IC
shuts down. Only when the IC temperature drops
to 135 OC, IC will restart.

 Output Short Load Protection

In CX7508, the output is sampled on INV pin and
then compared with a threshold of UVP (0.7V
typically) after an internal blanking time (10ms
Fig.3
typical). When sensed INV voltage is below 0.7V,
 Current Sensing and Leading Edge the IC will enter into Short Load Protection (SLP)
Blanking mode, in which t he IC will enter into auto
Cycle-by-Cycle current limiting is offered in recovery protection mode.
CX7508. The switch current is detected by a
sense resistor into the CS pin. An internal  VDD OVP and Zener Clamp
leading edge blanking circuit is built in. During When VDD voltage is higher than 30V (typical),
this blanking period (500ns, typical), the the IC will stop switching. This will cause VDD
cycle-by-cycle current limiting comparator is fall down to be lower than VDD_OFF (typical 9V)
disabled and cannot switch off the GATE driver. and then the system will restart up again. An
internal 34.5V (typical) zener clamp is integrated
 Audio Noise Free Operation to prevent the IC from damage.
CX7508 can provide audio noise free operation
from full loading to zero loading.  Soft Gate Drive
CX7508 has a soft totem-pole gate driver with
 Optimized Dynamic Response optimized EMI performance. An internal 16V
In CX7508, an optimized dynamic response clamp is added for MOSFET gate protection at
control is integrated to improve system dynamic higher than expected VDD input.

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 CX7508
High Precision CC/CV Primary-Side PWM Power Switch

Package Dimension

Dimensions In Millimeters Dimensions In Inches

Symbol
Min Max Min Max
A 3.710 5.334 0.146 0.210
A1 0.381 0.015
A2 3.175 3.600 0.125 0.142
B 0.350 0.650 0.014 0.026
B1 1.524 (BSC) 0.06 (BSC)
C 0.200 0.360 0.008 0.014
D 9.000 10.160 0.354 0.400
E 6.200 6.600 0.244 0.260
E1 7.320 7.920 0.288 0.312
e 2.540 (BSC) 0.1 (BSC)
L 2.921 3.810 0.115 0.150
E2 8.200 9.525 0.323 0.375

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