Datasheet LT1336 (Analog Devices) - 10

LT1336
operaTion (Refer to Functional Diagram)
The LT1336 incorporates two independent driver chan- ing supply. This allows the output to smoothly transition nels with separate inputs and outputs. The inputs are to 100% duty cycle. TTL/CMOS compatible; they can withstand input voltages An undervoltage detection circuit disables both channels as high as V+. The 1.4V input threshold is regulated and when V+ is below the undervoltage trip point. A separate has 300mV of hysteresis. Both channels are noninverting undervoltage detect block disables the high side channel drivers. The internal logic prevents both outputs from when VBOOST – VTSOURCE is below 9V. simultaneously turning on under any input conditions. When both inputs are high both outputs are actively The top and bottom gate drivers in the LT1336 each utilize held low. two gate connections: 1) a Gate Drive pin, which provides the turn-on and turn-off currents through an optional series An internal switching regulator permits smooth transi- gate resistor, and 2) a Gate Feedback pin which connects tion from PWM to DC operation. In PWM operation the directly to the gate to monitor the gate-to-source voltage. bootstrap capacitor is recharged each time Top Source pin goes low. As the duty cycle approaches 100% the output Whenever there is an input transition to command the pulse width becomes narrower and the time available to outputs to change states, the LT1336 follows a logical produce an elevated upper MOSFET gate supply becomes sequence to turn off one MOSFET and turn on the other. shorter than required. As the voltage across the bootstrap First, turn-off is initiated, then VGS is monitored until it capacitor drops below 10.6V, an inductor-based switching has decreased below the turn-off threshold, and finally regulator kicks in and takes over the charging of the float- the other gate is turned on.
applicaTions inForMaTion Deriving the Floating Supply
connected between V+ and the Boost pin is still needed to allow conventional bootstrapping of the bootstrap capaci- In a typical half-bridge driver like the LT1158 or the LT1160, tor when duty cycles are below 90%. the floating supply for the topside driver is provided by a bootstrap capacitor. This capacitor is recharged each The LT1336’s internal switching regulator can provide time its negative plate goes low in PWM operation. As enough charge to the bootstrap capacitor to allow the the duty cycle approaches 100% the output pulse width top driver to drive several power MOSFETs in parallel at becomes narrower and the time available to recharge the its maximum operating frequency. The regulated voltage bootstrap capacitor becomes shorter than required (1µs across VBOOST – VTSOURCE is 10.6V; when this voltage is to 2µs). For instance, at 100kHz and at 95% duty cycle the exceeded due to normal bootstrap action, the regulator output pulse width is only 0.5µs; clearly this is insufficient automatically shuts down. time to recharge the capacitor by bootstrapping. To get The switching regulator uses a hysteretic current mode around this problem, the LT1336 incorporates a switching control. This method of control is simple, inherently stable regulator to help recharge the bootstrap capacitor under and provides peak inductor current limit in every cycle. It is such extreme conditions. designed to run at a nominal frequency of around 700kHz The LT1336 provides al the necessary circuitry to construct which is 7× the maximum PWM operating frequency of the a boost or flyback switching regulator. This regulator can LT1336. Since the hysteretic current mode control has no charge the bootstrap capacitor when it cannot recharge internal oscillator, the frequency is determined by external by bootstrapping. This happens when nearing 100% duty conditions such as supply voltage and load currents and cycle in PWM applications. This is a worst-case condition external components such as inductor value and current because the bootstrap capacitor must still provide for the sense resistor value. gate charging current of the high side MOSFETs. A diode 1336fa 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Test Circuit Timing Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts