SG6848

OPERATION DESCRIPTION

SG6848 devices integrate many useful designs into one controller for low-power switch-mode power supplies. The following descriptions highlight some of the features of the SG6848 series.

Start-up Current

The start-up current is only 5uA. Low start-up current allows a start-up resistor with a high resistance and a low-wattage to supply the start-up power for the controller. A 1.5 M?, 0.25W, start-up resistor and a 10uF/25V V DD hold-up capacitor would be sufficient for an AC-to-DC power adapter with a wide input range (100V AC to 240V AC).

Operating Current

The operating current has been reduced to 2mA. The low operating current results in higher efficiency and reduces the V cc hold-up capacitance requirement. Green-Mode Operation

The proprietary green-mode function provides off-time modulation to linearly decrease the switching frequency under light-load conditions. On-time is limited to provide stronger protection against brownouts and other abnormal conditions. The feedback current, which is sampled from the voltage feedback loop, is taken as the reference. Once the feedback current exceeds the threshold current, the switching frequency starts to decrease. This green-mode function dramatically reduces power consumption under light-load and zero-load conditions. Power supplies using the SG6848 can easily meet even the strictest regulations regarding standby power consumption.

Oscillator Operation

A resistor connected from the RI pin to ground will generate a constant current source for the SG6848. This current is used to charge an internal capacitor. The charge-time determines the internal clock speed and the switching frequency. Increasing the resistance will reduce the amplitude of the input current and reduce the switching frequency. A 95k? resistor R i results in a 50uA constant current I i and a 70kHz switching frequency. The relationship between R i and the switching frequency is:

)

(kHz

)

(k

R

6650

I

PWM

f

?

=

The range of the oscillation frequency is designed to be within 50kHz ~ 100kHz.

Leading-Edge Blanking

Each time the power MOSFET is switched on, a turn-on spike will inevitably occur at the sense-resistor. To avoid premature termination of the switching pulse, a 250 nsec leading-edge blanking time is built in. Conventional RC filtering can therefore be omitted. During this blanking period, the current-limit comparator is disabled and it cannot switch off the gate driver.

Constant Output Power Limit

When the SENSE voltage across the sense resistor R s reaches the threshold voltage (around 1.0V), the output GATE drive will be turned off following a short propagation delay t D. This propagation delay will introduce an additional current proportional to t D*V in/L p. The propagation delay is nearly constant regardless of the input line voltage V IN. Higher input line voltages will result in larger additional currents. At high input line voltages, the output power limit will be higher than at low input line voltages.

To compensate for this output power limit variation across a wide AC input range, the threshold voltage is adjusted by adding a positive ramp. This ramp signal rises from 0.85V to 1.0V, and then flattens out at 1.0V. A smaller threshold voltage forces the output GATE drive to terminate earlier. This reduces the total PWM turn-on time and makes the output power equal to that of low line input. This proprietary internal compensation ensures a constant output power limit for a wide AC input voltage range (90VAC to 264VAC).

Under Voltage Lockout (UVLO)

The turn-on and turn-off thresholds of the SG6848 are fixed internally at 16.3V/11.7V. During start-up, the hold-up capacitor must be charged to 16.3V through the start-up resistor, so that the SG6848 will be enabled. The hold-up capacitor will continue to supply V DD until power can be delivered from the auxiliary winding of the main transformer. V DD must not drop below 11.7V during this start-up process. This UVLO hysteresis window ensures that hold-up capacitor will be adequate to supply V DD during start-up.

Gate Output

The SG6848 BiCMOS output stage is a fast totem pole gate driver. Cross conduction has been avoided to minimize heat dissipation, increase efficiency, and enhance reliability. The output driver is clamped by an internal 15V Zener diode in order to protect power MOSFET transistors against undesired over-voltage gate signals.

Built-in Slope Compensation

The sensed voltage across the current sense resistor is used for current mode control and pulse-by-pulse current limiting. Built-in slope compensation will improve stability and prevent sub-harmonic oscillations due to peak-current mode control. The SG6848 has a synchronized, positively-sloped ramp built-in at each switching cycle. The slope of the ramp is:

(max)

33

.0

Duty

Duty

×

Noise Immunity

Noise from the current sense or the control signal can cause significant pulse width jitter, particularly in continuous-conduction mode. While slope compensation helps alleviate these problems, further precautions should still be taken. Good placement and layout practices should be followed. Avoiding long PCB traces and component leads, locating compensation and filter components near the SG6848, and increasing the power MOS gate resistance is advised.

DISCLAIMERS

LIFE SUPPORT

System General’s products are not designed to be used as components in devices intended to support or sustain human life. Use of System General’s products in components intended for surgical implant into the body, or other applications in which failure of System General’s products could create a situation where personal death or injury may occur, is not authorized without the express written approval of System General’s Chief Executive Officer. System General will not be held liable for any damages or claims resulting from the use of its products in medical applications. MILITARY

System General's products are not designed for use in military applications. Use of System General’s products in military applications is not authorized without the express written approval of System General’s Chief Executive Officer. System General will not be held liable for any damages or claims resulting from the use of its products in military applications.

RIGHT TO MAKE CHANGES

System General reserves the right to change this document and/or this product without notice. Customers are advised to consult their System General sales representative before ordering.