Datasheet LT3002 (Analog Devices) - 5

LT3002
PIN FUNCTIONS EN/UVLO (Pin 1):
Enable/Undervoltage Lockout. The
SW (Pin 5):
Drain of the Internal DMOS Power Switch. EN/UVLO pin is used to enable the LT3002. Pull the pin Minimize trace area at this pin to reduce EMI and below 0.3V to shut down the LT3002. This pin has an voltage spikes. accurate 1.214V threshold and can be used to program a
R
V
FB (Pin 6):
Input Pin for External Feedback Resistor. IN undervoltage lockout (UVLO) threshold using a resis- Connect a resistor from this pin to the transformer pri- tor divider from VIN to ground. A 2.5µA current hysteresis mary SW pin. The ratio of the R al ows the programming of V FB resistor to the RREF IN UVLO hysteresis. If neither resistor, times the internal voltage reference, determines function is used, tie this pin directly to VIN. the output voltage (plus the effect of any non-unity trans-
INTVCC (Pin 2):
Internal 3V Linear Regulator Output. The former turns ratio). Minimize trace area at this pin. INTVCC pin is supplied from VIN and powers the inter-
R
nal control circuitry and gate driver. Do not overdrive the
REF (Pin 7):
Input Pin for External Ground Referred Reference Resistor. The resistor at this pin should be in INTVCC pin with any external supply, such as a third wind- the range of 10k, but for convenience in selecting a resis- ing supply. Locally bypass this pin to ground with a mini- tor divider ratio, the value may range from 9.09k to 11.0k. mum 1µF ceramic capacitor.
TC (Pin 8):
Output Voltage Temperature Compensation.
VIN (Pin 3):
Input Supply. The VIN pin supplies current to The voltage at this pin is proportional to absolute tem- the internal circuitry and serves as a reference voltage for perature (PTAT) with temperature coefficient equal the feedback circuitry connected to the RFB pin. Locally to 3.35mV/°K, i.e., equal to 1V at room temperature bypass this pin to ground with a capacitor. 25°C. The TC pin voltage can be used to estimate the
GND (Pin 4, Exposed Pad Pin 9):
Ground. The exposed LT3002 junction temperature. Connect a resistor from pad provides both electrical contact to ground and good this pin to the RREF pin to compensate the output diode thermal contact to the printed circuit board. Solder the temperature coefficient. exposed pad directly to the ground plane.
OPERATION
The LT3002 is a current mode switching regulator IC Circuits employing extra transformer windings also exhibit designed specially for the isolated flyback topology. The deficiencies, as using an extra winding adds to the trans- key problem in isolated topologies is how to commu- former’s physical size and cost, and dynamic response nicate the output voltage information from the isolated is often mediocre. secondary side of the transformer to the primary side The LT3002 samples the isolated output voltage through for regulation. Historically, opto-isolators or extra trans- the primary-side flyback pulse waveform. In this man- former windings communicate this information across ner, neither opto-isolator nor extra transformer winding the isolation boundary. Opto-isolator circuits waste output is required for regulation. Since the LT3002 operates power, and the extra components increase the cost and in either boundary conduction mode or discontinuous physical size of the power supply. Opto-isolators can also conduction mode, the output voltage is always sampled cause system issues due to limited dynamic response, on the SW pin when the secondary current is zero. This nonlinearity, unit-to-unit variation and aging over lifetime. method improves load regulation without the need of external load compensation components. Rev. 0 For more information www.analog.com 5 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Applications Information Package Description