Datasheet LT3956 (Analog Devices) - 9

LT3956 operaTion The LT3956 is a constant-frequency, current mode tor. Likewise, at an ISP/ISN common mode voltage less converter with a low side N-channel MOSFET switch. than 3V, the difference between ISP and ISN is monitored The switch and PWMOUT pin drivers, and other chip to determine if the output is in a short-circuit condition. If loads, are powered from INTVCC, which is an internally the difference between ISP and ISN is greater than 335mV regulated supply. In the discussion that follows, it will be (typical), the SR latch will be reset regardless of the PWM helpful to refer to the Block Diagram of the IC. In normal comparator. These functions are intended to protect the operation, with the PWM pin low, the power switch is power switch, as well as various external components in turned off and the PWMOUT pin is driven to GND, the the power path of the DC/DC converter. VC pin is high impedance to store the previous switching In voltage feedback mode, the operation is similar to that state on the external compensation capacitor, and the ISP described above, except the voltage at the V and ISN pin bias currents are reduced to leakage levels. C pin is set by the amplified difference of the internal reference of 1.25V When the PWM pin transitions high, the PWMOUT pin (nominal) and the FB pin. If FB is lower than the reference transitions high after a short delay. At the same time, the voltage, the switch current will increase; if FB is higher internal oscillator wakes up and generates a pulse to set than the reference voltage, the switch demand current the PWM latch, turning on the internal power MOSFET will decrease. The LED current sense feedback interacts switch. A voltage input proportional to the switch current, with the FB voltage feedback so that FB will not exceed sensed by an internal current sense resistor, is added to the internal reference and the voltage between ISP and a stabilizing slope compensation ramp and the resulting ISN will not exceed the threshold set by the CTRL pin. switch-current sense signal is fed into the positive termi- For accurate current or voltage regulation, it is necessary nal of the PWM comparator. The current in the external to be sure that under normal operating conditions, the inductor increases steadily during the time the switch is appropriate loop is dominant. To deactivate the voltage on. When the switch-current sense voltage exceeds the loop entirely, FB can be connected to GND. To deactivate output of the error amplifier, labeled VC, the latch is reset the LED current loop entirely, the ISP and ISN should be and the switch is turned off. During the switch off phase, tied together and the CTRL input tied to V the inductor current decreases. At the completion of each REF . oscillator cycle, internal signals such as slope compensa- Two LED specific functions featured on the LT3956 are tion return to their starting points and a new cycle begins controlled by the voltage feedback pin. First, when the with the set pulse from the oscillator. FB pin exceeds a voltage 50mV lower (–4%) than the FB regulation voltage, the pull-down driver on the VMODE pin Through this repetitive action, the PWM control algorithm is activated. This function provides a status indicator that establishes a switch duty cycle to regulate a current or the load may be disconnected and the constant-voltage voltage in the load. The VC signal is integrated over many feedback loop is taking control of the switching regulator. switching cycles and is an amplified version of the differ- When the FB pin exceeds the FB regulation voltage by 60mV ence between the LED current sense voltage, measured (5% typical), the PWMOUT pin is driven low, ignoring the between ISP and ISN, and the target difference voltage state of the PWM input. In the case where the PWMOUT set by the CTRL pin. In this manner, the error amplifier pin drives a disconnect NFET, this action isolates the sets the correct peak switch-current level to keep the LED load from GND, preventing excessive current from LED current in regulation. If the error amplifier output damaging the LEDs. If the FB input exceeds the overvolt- increases, more current is demanded in the switch; if it age threshold (1.31V typical), then an externally driven decreases, less current is demanded. The switch current overvoltage event may have caused the FB pin to be too is monitored during the on-phase and is not allowed to high and the VMODE pull-down will be deactivated until exceed the current limit threshold of 3.9A (typical). If the the FB pin drops below the overvoltage threshold. SW pin exceeds the current limit threshold, the SR latch is reset regardless of the output state of the PWM compara- 3956f Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Typical Application Related Parts