Datasheet LTC3783 (Analog Devices) - 9

LTC3783
OPERATION Main Control Loop
For constant current/constant voltage regulation operation The LTC3783 is a constant frequency, current mode con- (defined by VFBP > 2.5V), please refer to the Block Diagram troller for PWM LED as well as DC/DC boost, SEPIC and of the IC and Figure 11. Loop operation is similar to the flyback converter applications. In constant current LED voltage feedback, except FBP and FBN now sense the applications, the LTC3783 provides an especially wide PWM voltage across sense resistor RL in series with the load. dimming range due to its unique switching scheme, which The ITH pin now represents the error from the desired dif- allows PWM pulse widths as short as several converter ferential set voltage, from 10mV to 100mV, for ILIM values switching periods. of 0.123V to 1.23V. That is, with VILIM = 1.23V, the loop will regulate such that VFBP – VFBN = 100mV; lower values For voltage feedback circuit operation (defined by VFBP ≤ of ILIM attenuate the difference proportionally. PWMIN is 1.23V), please refer to the Block Diagram of the IC and the still functional as above, but will only work properly if load Typical Application on the first page of this data sheet. In current can be disconnected by the PWMOUT signal. normal operation with PWMIN high, the power MOSFET is turned on (GATE goes high) when the oscillator sets In constant current/constant voltage operation, the OV/FB the PWM latch, and is turned off when the ITRIP current pin becomes a voltage feedback pin, which causes the loop comparator resets the latch. Based on the error voltage to regulate such that VOV/FB = 1.23V, provided the above represented by (V current-sense voltage is not reached. In this way, the loop FBP – VFBN), the error amplifier output signal at the I regulates either voltage or current, whichever parameter TH pin sets the ITRIP current comparator input threshold. When the load current increases, a fall in the hits its preset limit first. FBN voltage relative to the reference voltage at FBP causes The nominal operating frequency of the LTC3783 is pro- the ITH pin to rise, causing the ITRIP current comparator grammed using a resistor from the FREQ pin to ground to trip at a higher peak inductor current value. The average and can be controlled over a 20kHz to 1MHz range. In inductor current will therefore rise until it equals the load addition, the internal oscillator can be synchronized to an current, thereby maintaining output regulation. external clock applied to the SYNC pin and can be locked When PWMIN goes low, PWMOUT goes low, the I to a frequency between 100% and 130% of its nominal TH switch opens and GATE switching is disabled. Lowering PWMOUT value. When the SYNC pin is left open, it is pulled low by and disabling GATE causes the output capacitor C an internal 100k resistor. With no load, or an extremely OUT to hold the output voltage constant in the absence of load light one, the controller will skip pulses in order to maintain current. Opening the I regulation and prevent excessive output ripple. TH switch stores the correct load current value on the ITH capacitor CITH. As a result, when The RUN pin controls whether the IC is enabled or is in a PWMIN goes high again, both ITH and VOUT are instantly low current shutdown state. A micropower 1.248V refer- at the appropriate levels. ence and RUN comparator allow the user to program the In voltage feedback operation, an overvoltage compara- supply voltage at which the IC turns on and off (the RUN tor, OV, senses when the OV/FB pin exceeds the reference comparator has 100mV of hysteresis for noise immunity). voltage by 7% and provides a reset pulse to the main RS With the RUN pin below 1.248V, the chip is off and the latch. Because this RS latch is reset-dominant, the power input supply current is typically only 20µA. MOSFET is actively held off for the duration of an output overvoltage condition. 3783fb 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Package Description Related Parts