LT1681 UUUPI FU CTIO S The error amplifier can also be configured as a BG (Pin 16): Bottom-Side Primary Switch/Forward Switch transimpedance amplifier for use in secondary-side con- Output Driver. Pin can be connected directly to gate of troller applications. (See Applications Information section primary bottom-side and forward switches if small FETs for configuration and compensation details) are used (CGATE total < 5000pF), however, the use of a gate SENSE (Pin 11): Current Sense Amplifier (CSA) Nonin- drive buffer is recommended for peak efficiencies. verting Input. Current is monitored via a ground refer- The BG output is enabled at the start of each oscillator enced current sense resistor, typically in series with the cycle in phase with the TG pin but is timed to “lag” the TG source of the bottom-side switch FET. Internal limit cir- output during turn-on and “lead” the TG output during cuitry provides for a maximum peak value of 150mV across turn-off. These delays force the concentration of transi- the sense resistor during normal operation. tional losses onto the bottom-side primary switch. IMAX (Pin 12): Primary Current Runaway Protection. The BLKSENS (Pin 17): Blanking Sense Input. The current IMAX pin is used to detect primary-side switch currents sense function (via SENSE pin) is disabled while the and shuts down the primary switches if a current runaway BLKSENS pin is below 5V. BLKSENS is typically con- condition is detected. The IMAX function is not disabled nected to the gate of the bottom-side primary switch during the current sense blanking interval. The pin is MOSFET. typically connected to the primary bottom-side switch BSTREF (Pin 18): V source and monitors switch current via a ground-refer- BST Supply Reference. Typically con- nects to source of topside external power FET switch. enced current sense resistor. If the pin voltage exceeds 360mV, LT1681 switching function is disabled in 130ns. TG (Pin 19): Topside (Boosted) Primary Output Driver. Pin Exceeding the IMAX threshold also triggers a soft-start can be connected directly to gate of primary topside reset, resulting in a graceful recovery from a current switch if small FETs are used (CGATE < 5000pF), however, runaway event. For single-sense resistor systems, this pin the use of a gate drive buffer is recommended for peak can be shorted to SENSE for protection during the blank- efficiencies. ing interval or shorted to SGND if not used. VBST (Pin 20): Topside Primary Driver Bootstrapped Sup- SG (Pin 13): Synchronous Switch Output Driver. This pin ply. This “boosted” supply rail is referenced to the BSTREF can be connected directly to the gate of the synchronous pin. switch if small FETs are used (CGATE < 5000pF), however, Supply voltage is maintained by a bootstrap capacitor tied the use of a gate drive buffer is recommended for peak from the VBST pin to the boosted supply reference (BSTREF) efficiencies. pin. The charge on the capacitor is refreshed each switch The SG pin output is synchronized and out-of-phase with cycle through a Schottky diode connected from the VCC the BG output. The control timing of the SG output causes supply (cathode) to the VBST pin (anode). The bootstrap its transition to “lead” the primary switch path during turn- capacitor (CBOOST) must be at least 100 times greater than on by 150ns. the total load capacitance on the TG pin. A capacitor in the range of 0.1µF to 1µF is generally adequate for most VCC (Pin 14): IC Local Power Supply Input. Bypass with a applications. The bootstrap diode must have a reverse- capacitor at least 10 times greater than C5VREF to PGND. breakdown voltage greater than the converter V The LT1681 incorporates undervoltage lockout that dis- IN. The LT1681 supports operational V ables switching functions if V BST supply voltages up to CC is below 8.4V. The LT1681 90V (absolute maximum) referenced to ground. supports operational VCC power supply voltages from 9V Undervoltage lockout disables the topside switch until to 18V (20V absolute maximum). VBST-BSTREF > 7.0V for start-up protection of the topside PWRGND (Pin 15): Output Driver Ground Reference. switch. Connect through low impedance trace to VIN decoupling capacitor. 1681f 8
