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Feb 2, 2015

Motorola Car Battery Power Adapter (6.4 V 1 A) Buck Converter Power Supply (2005)

SYN9324A, 2005 Buck Converter, Car Battery to 6.4 VDC 1A

Motorola Phone Car Adapter Power Supply Reverse Engineerging

Motorola SYN9324 Car Adapter ($ 15.95)

Motorola charging adapter SYN9324 Specs

  • Built &  Designed specifically for your New Motorola C139/ C155/ C168i/ V150/ V151/ V170 / V171 / V173/ V176/ C257/ C261 cell phone.
  • Dependable and portable in-vehicle battery charging accessory.
  • SYN0934 Motorola car battery charger accessory is designed to rapidly charge your cellular battery in approximately two to three hours.
  • Simply plug it into your car's vehicle power socket and connect it to your motorola phone.
  • With this Motorola car charger enjoy unlimited talk and standby time while your cellular phone is charging!
  • OEM Motorola vehicle charging adapter quickly plugs into your car 12VDC cigarette lighter adapter for a rapid battery charge to keep your Motorola cell phone battery in top operating condition.
  • This Motorola car charger SYN9324, Original Motorola Accessory immediately operates your New Motorola C139/ C155/ C168i/ V150/ V151/ V170 / V171 / V173/ V176/ C257/ C261 when attached to a rechargeable battery.

Motorola C139 Phone

Best feature of this circuit is its wide operating voltage range allowing the car battery voltage to be as low as 7 V. The fuse is a PTC temperature controlled fuse resetting it self automatically after overloading thus avoiding the need to manually replace any fuses.

C139 Charger Connector

The input is well protected using a varistor or transient suppressor at 33 V (ZNR1). The PWM integrated circuit in this converter is the TL594(IN) with an industrial temperature range so it will survive temperatures below -20 degrees C. The 594 is pin compatible with the widely in PC power supplies used 494 PWM IC with some minor additionally features (under voltage switch off).

TL594 Block Diagram

494 Block Diagram

The advantage of this 594 circuit is that it measures the current consumption in the positive voltage rail, not in the negative rail as so many 494 PC power supply circuits do. To accomplish this it uses an additional differential operational amplifier circuit based on a (LM)358 dual op amp (U2). The following figure shows the buck converter's schematic.

Motorola SYN9324 Car Charger Schematic

The second op amp in the 358 IC is just used as a control signal voltage level comparator to drive the general purpose NPN transistor to lower the current limit when the phone needs less current. When the CURRENT CNTRL signal is low the power supply gives the full amperage limiting it at about 1 A and when it is hight only about 0.5 A is given. The resistors are so chosen that the Vref (5V) from the PWM chip is either divided to 2.06 or 0.90 Volts and then fed into the negative input of the error amplifier 2 in the PWM controller. Since the differential amplifier in the 358 IC rejects the B potential and multiplies the result with 10 the small potential over the resistors R17 and R18 are measured. This potential can be 0.206 or 0.09 Volts. The resulting limiting current can be calculated as follows:

Iout = Ux/((R17*R18)/(R17 + R18))
     = 0.206/0.2222 = 0.93 A
     = 0.090/0.2222 = 0.41 A

TL594 is pin to pin compatible with the very well known 494 circuit used in many PC power supply circuits. This circuit can be used as a buck or boost converter or a push- pull converter. The output function can be controlled via the OC pin. In SYN9324 circuit both driver transistors work parallel boosting the drive capability of the circuit.

Inside SYN9324

There are two closed loops in the PWM controller. One to keep the output voltage at the nominal 6.44 V and another to limit the output current to either 1 A or 0.5 A as described before. The 358 op amp is used to amplify the current sense voltage about 0.1 or 0.2 V by 10 and to level shift it down for the 594 current sense op amp.

Component side

Since the PWM controller circuit is a closed loop circuit it oscillates very easy at higher frequencies if the amplification at high frequencies is not limited. Limiting the amplification at higher frequencies is done by using the feedback output at the compensation pin 3.

At lower frequencies the capacitors C3 and C4 have a larger impedance thus yielding a higher amplification at lower frequencies. At high frequencies the capacitors are short circuits and only the series resistors R6 and R16 set the amplification. Thus dumping the oscillations at high frequencies.

Bottom side with SM Components

A note worth is the absence of any bypass ceramic capacitors for the electrolytic capacitors which are usual employed in circuits with integrated circuits. And I would add one or two of those in the circuit to keep the power rails clean.

Reverse engineered board


Motorola C139 video


TL594 PWM Controller IC
LM358 op amp
2SA1012 PNP Power Switch Transistor
SB360 fast diode 3A 60V
ZR24 General Purpose SMD Transistor
PTC Fuse 5A
CNR 07D330K 33V Transient Suppressor

(Please use Google to find the latest supplies for the datasheets)

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