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TSM006 Datasheet(PDF) 6 Page - STMicroelectronics |
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TSM006 Datasheet(HTML) 6 Page - STMicroelectronics |
6 / 13 page TSM006 Functional Description 6/13 4 Functional Description TSM006: PWM Controller IC. UVLO function The Under Voltage Lock Out function disables the whole device when supply voltage is lower than the threshold. Vref block The Vref block provides a 5V reference voltage. An internal Vref status signal is active when Vref is lower than 4.7V and is used to drive the output driver low when Vref is not valid. Current sense input A voltage proportional to the output inductor current is applied to the CS pin. The control IC uses this information to perform current mode control. The PWM function will be stopped if the CS pin voltage is greater than 1.0V. Current leading edge blanking An internal delay is built into the IC to mask the first 100ns of the current sense signal. This delay is made of a capacitor charged with a current source. The capacitor is discharged when CT reaches its maximum level. COMP input This pin is connected to the current comparator for current mode control. The pin should be connected to the collector (primary side) of an optocoupler which anode (secondary side) is driven by the output of error amplifier. The COMP input is used to set the reference level for the current sense comparator. The current sense threshold is set to (Vcomp - 2 * Vbe) / 3. During the soft start period, COMP voltage is clamped to the SS pin plus two Vbe voltage. Startup latch The startup latch is set when the IC exits from standby mode or UVLO state. It is reset when the CT capacitor is discharged for the first time. Output driver The OUT totem pole output is capable to sink and source more than 1.0A (peak) in order to direct drive a power MOSFET. Oscillator A capacitor from the RT/CT pin to GND and a resistor to the VREF set the oscillating frequency. The maximum duty cycle at the OUT pin is limited at 77%. Frequency modulation A FM generator adds a small amount of jitter on the oscillator frequency in a way that reduces the conducted and radiated EMI. The FM frequency is set by an external capacitor connected to the FM pin. Slope compensation A buffered Rt/Ct voltage is brought to the Cslope pin. This signal is used to provide the necessary slope compensation. Soft start A capacitor from the SS pin to GND provides the soft start function. The capacitor starts to charge when VIN reaches the UVLO threshold and Vref is good. The soft start block enables the IC to start with a progressive PWM duty cycle. The soft start comparator drives the output driver low when the SS pin voltage is greater than the CT pin voltage minus one Vbe voltage. During soft start, the COMP pin voltage is clamped to the SS pin voltage plus two Vbe voltage, limiting the maximum peak current. External reference pin An external resistor at REX pin sets the internal current reference. |
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