USRB04

Preliminary

CMOS IC

UNISONIC TECHNOLOGIES CO., LTD

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www.unisonic.com.tw

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OPERAT I ON DESCRI PT I ON

USRB04

is a cost effective PWM power switch optimized for off-line non-isolated buck or buk-boost applications

for small home appliances and linear regulator replacement. It operates in current mode and regulates output

voltage with dedicated features. High integration can afford low cost and component count solution.

Startup Current and Start up Control

starts up quickly. A large value startup resistor can therefore be used to minimize the power loss in application.

Operating Current

The Operating current of USRB04 is as low as 1.2mA (typical). Good efficiency is achieved with the low operation

current together with ‘Multi-mode’ control features.

PWM operation

The maximum switching frequency of USRB04 is internally fixed at 40KHz (typical). No external frequency setting

components are required for PCB design simplification.

At light load or zero load condition, most of the power dissipation in a switching mode power supply is from

switching loss on the BJT. The magnitude of power loss is in proportion to the switching frequency. Lower switching

frequency leads to the reduction on the power loss and thus conserves the energy. The frequency reduction and

burst mode operation are implemented to achieve high efficiency at light load. The minimum switching frequency is

20KHz (typical).

Frequency shuffling fro EMI improvement

The frequency shuffling (switching frequency modulation) is implemented in USRB04. The oscillation frequency is

modulated so that the tone energy is spread out. The spread spectrum minimizes the conduction band EMI and

therefore eases the system design.

Soft Start

USRB04

features an internal 256 cycles (typical) soft start to soften the electrical stress occurring in the power

frequency is gradually increased from 10KHz to 40 KHz. Every restart up is followed by a soft start.

Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in USRB04 current mode PWM control. The switch current is detected by

a sense resistor into the CS pin. An internal leading edge blanking circuit chops off the sensed voltage spike at initial

internal BJT on state so that the external RC filtering on sense input is no longer needed. The PWM duty cycle is

determined by the current sense input voltage and the EA output voltage.

Constant Power Operation Mode

output current, output voltage declines with output power is kept in a about constant level. This is suitable for peak

current application.

Base Driver

Protection Control

Good power supply system reliability is achieved with its rich protection features including cycle-by-cycle current