 |
 |
|
| SC1102_06 |
|
|||||||||||||||||
 SEMTECH
|
| |||||||||||||||||
|
6 page 
6 2006 Semtech Corp. www.semtech.com POWER MANAGEMENT SC1102/SC1102A Soft Start Initially, SS/SHDN sources 10µA of current to charge an external capacitor. The outputs of the error amplifiers are clamped to a voltage proportional to the voltage on SS/SHDN. This limits the on-time of the high-side MOSFETs, thus leading to a controlled ramp-up of the output voltages. R DS(ON) Current Limiting The current limit threshold is set by connecting an exter- nal resistor from the V CC supply to OCSET. The voltage drop across this resistor is due to the 200µA internal sink sets the voltage at the pin. This voltage is compared to the voltage at the PHASE node. This comparison is made only when the high-side drive is high to avoid false current limit triggering due to uncontributing measure- ments from the MOSFETs off-voltage. When the voltage at PHASE is less than the voltage at OCSET, an overcurrent condition occurs and the soft start cycle is initiated. The synchronous switch turns off and SS/SHDN starts to sink 2µA. When SS/SHDN reaches 0.8V, it then starts to source 10µA and a new cycle begins. Hiccup Mode During power up, the SS/SHDN pin is internally pulled low until VCC reaches the undervoltage lockout level of 4.2V. Once V CC has reached 4.2V, the SS/SHDN pin is released and begins to source 10µA of current to the external soft-start capacitor. As the soft-start voltage rises, the output of the internal error amplifier is clamped to this voltage. When the error signal reaches the level of the internal triangular oscillator, which swings from 1V to 2V at a fixed frequency of 200kHz/500kHz, switching occurs. As the error signal crosses over the oscillator signal, the duty cycle of the PWM signal continues to in- crease until the output comes into regulation. If an over- current condition has not occurred the soft-start voltage will continue to rise and level off at about 2.2V. Applications Information (Cont.) An over-current condition occurs when the high-side drive is turned on, but the PHASE node does not reach the voltage level set at the OCSET pin. The PHASE node is sampled only once per cycle during the valley of the tri- angular oscillator. Once an over-current occurs, the high- side drive is turned off and the low-side drive turns on and the SS/SHDN pin begins to sink 2uA. The soft-start voltage will begin to decrease as the 2uA of current dis- charges the external capacitor. When the soft-start volt- age reaches 0.8V, the SS/SHDN pin will begin to source 10uA and begin to charge the external capacitor causing the soft-start voltage to rise again. Again, when the soft- start voltage reaches the level of the internal oscillator, switching will occur. If the over-current condition is no longer present, normal operation will continue. If the over-current condition is still present, the SS/SHDN pin will again begin to sink 2uA. This cycle will continue indefinitely until the over- current condition is removed. In conclusion, below is shown a typical “12V Application Circuit” which has a BSTH voltage derived by bootstrapping input voltage to the PHASE node through diode D1. This circuit is very useful in cases where only input power of 12V is available. In order to prevent substrate glitching, a small-signal di- ode should be placed in close proximity to the chip with cathode connected to PHASE and anode connected to PGND. |
