Switcher IC. RM3253S Datasheet
Off Line AC/DC Switcher IC
The RM3253S series switcher ICs cost effectively replace all power supplies up to 5W output power based on
unregulated isolated linear transformer (50/60HZ). Unlike conventional PWM (pulse width modulation)
controllers, they regulate the output voltage in a new method of off-time modulation control. The controller
consists of a VCO (voltage control oscillator), Sense and logic circuit, VDD pin, under-voltage lockout circuit,
protection for over-voltage, current limited circuit, leading edge blanking, over load protection and fault condition
auto-restart. They are ideal low power AC/DC adapter/charger solution for portable devices,
• Built-in 800V high voltage BJT and minimal external componets
• Low start-up current: Typ.15 μA Max.25 μA
• UVLO(under voltage lockout):Typ.12V start-up Typ.6V shut-down
• Low operating current:Typ.2 mA Max.5 mA
• LEB(leading edge blanking): Typ.300ns
• OVP(over voltage protection): Typ.28 V
• Operation Frequency: Typ.54KHZ(VDD=21V)
• Output Driver Ability: Typ.15 mA
• OLP(over load protection)
• Universal AC input range
• Constant voltage and constant current
• SOP-8 package.
•AC/DC adapter/charger for cell/cordless phones, digital cameras, MP3/portable audio devices etc
• LED Lighting
• Replacements for linear transformers and RCC SMTPS
Pin Connection (Top View)
PIN 1: VDD
PIN 2: NC
PIN 3: Sense
PIN 4: OE
PIN 5、6: OC
PIN 7、8: GND
Current sensor,it senses the voltage via a sensed resistor
Emmiter Pin of the power BJT switch.
Output Pin( Connect to switching transformer)
1. Under Voltage Lockout
The turn-on and turn-off thresholds of the RM3253S are fixed internally at 12V/6V. During start-up, the hold-up
capacitor must be charger to 12V through the start-up resistor, so that the RM3253S can be enabled. The hold-up
capacitor will continue to supply VDD until power can be delivered from the auxiliary winding of the main
ransformer, When VDD drop below 6V, IC will be shut-down, and have to start-up again. This UVLO hysteretic
window ensures that RM3253S can start-up very well.
2. Leading Edge Blanking
When the power BJT is switched on, a voltage spike will inevitably occur at the sense pin. To avoid premature
termination of the switching pulse, a 300 ns leading-edge blanking time is built in RM3253S. Conventional RC
filtering can be omitted. During this blanking period, the current-limit comparator is disabled and it cannot switch
off the gate driver.
3. Voltage Control Oscillator
The sense circuit of RM3253S series will sense the voltage of VDD pin (also the power pin of RM3253S) and then
control the frequency of the oscillator. The max frequency of RM3253S series is set to the rated output power
point,for example RM3253S is set to 54 KHZ Two signals are generated from the oscillator: the maximum duty
cycle signal (DC max) and the clock signal that indicates the beginning of each switching cycle.
4. Sense Circuit
A resistor voltage divided circuit is connected between the VDD pin and GND pin in order to sample the
feed-back voltage at VDD pin. When the voltage of VDD pin exceeds 21V, IC will come into constant voltage
control mode,shut- down the output pin and start a bypass circuit. This bypass circuit is linearly controlled by the
feedback voltage. When the voltage of VDD pin falls below 21V, the off-time linearly increases in order to have a
constant current output.
5. Gate Output
Unlike other chips, RM3253S used a constant current circuit to drive the output BJT. The output current is fixed at
15mA, select the appropriate HFE of BJT could have a different output power.
6. Fault Condition Auto-restart
In the event of a fault condition such as output over-load or short circuit, RM3253S series enter into auto-restart
operation. It is using the turn-on and turn-off threshold again. RM3253S series are working in start mode to protect
the system from damage caused by the fault condition.
7. Current Limit
The current through the power BJT is detected by the current limit circuit. Then the current flows through the
sense-resistor and makes the voltage on Sense pin ramp up. And when this voltage exceeds the internal threshold
(Vlimit), the power BJT is turned off immediately.