Datasheet LTC3703 (Analog Devices) - 10

LTC3703
operaTion (Refer to Functional Diagram)
side MOSFETs can occur. To prevent this from occurring, duty cycle control set to 0%. As CSS continues to charge, the bottom driver return is brought out as a separate pin the duty cycle is gradually increased, allowing the output (BGRTN) so that a negative supply can be used to reduce voltage to rise. This soft-start scheme smoothly ramps the the effect of the Miller pull-up. For example, if a –2V sup- output voltage to its regulated value with no overshoot. ply is used on BGRTN, the switch node dV/dt could pull The RUN/SS voltage will continue ramping until it reaches the gate up 2V before the VGS of the bottom MOSFET has an internal 4V clamp. Then the MIN feedback comparator more than 0V across it. is enabled and the LTC3703 is in full operation. When the V RUN/SS is low, the supply current is reduced to 50µA. DRVCC IN LTC3703 DRV DB + V CC OUT BOOST CIN TG CB 0V MT NORMAL OPERATION SHUTDOWN START-UP CURRENT SW L LIMIT VOUT MIN COMPARATOR ENABLED 4V OUTPUT VOLTAGE BG + IN REGULATION MB COUT 3V RUN/SS SOFT-STARTS BGRTN VRUN/SS OUTPUT VOLTAGE AND 1.4V INDUCTOR CURRENT 3703 F03 0V TO –5V 0.9V MINIMUM
Figure 3. Floating TG Driver Supply and Negative BG Return
0V DUTY CYCLE LTC3703 POWER ENABLE DOWN MODE 3703 F04
Constant Frequency Figure 4. Soft-Start Operation in Start-Up and Current Limit
The internal oscillator can be programmed with an external resistor connected from fSET to ground to run between
Current Limit
100kHz and 600kHz, thereby optimizing component size, The LTC3703 includes an onboard current limit circuit that efficiency, and noise for the specific application. The internal limits the maximum output current to a user-programmed oscillator can also be synchronized to an external clock level. It works by sensing the voltage drop across the applied to the MODE/SYNC pin and can lock to a frequency bottom MOSFET and comparing that voltage to a user- in the 100kHz to 600kHz range. When locked to an external programmed voltage at the I clock, pulse-skip mode operation is automatically disabled. MAX pin. Since the bottom MOSFET looks like a low value resistor during its on-time, Constant frequency operation brings with it a number of the voltage drop across it is proportional to the current benefits: inductor and capacitor values can be chosen for flowing in it. In a buck converter, the average current in a precise operating frequency and the feedback loop can the inductor is equal to the output current. This current be similarly tightly specified. Noise generated by the circuit also flows through the bottom MOSFET during its on-time. will always be at known frequencies. Subharmonic oscil- Thus by watching the drain-to-source voltage when the lation and slope compensation, common headaches with bottom MOSFET is on, the LTC3703 can monitor the output constant frequency current mode switchers, are absent in current. The LTC3703 senses this voltage and inverts it to voltage mode designs like the LTC3703. allow it to compare the sensed voltage (which becomes more negative as peak current increases) with a positive
Shutdown/Soft-Start
voltage at the IMAX pin. The IMAX pin includes a 12µA The main control loop is shut down by pulling RUN/SS pull-up, enabling the user to set the voltage at IMAX with pin low. Releasing RUN/SS allows an internal 4µA current a single resistor (RIMAX) to ground. See the Current Limit source to charge the soft-start capacitor, CSS. When CSS Programming section for RIMAX selection. reaches 0.9V, the main control loop is enabled with the 3703fc 10 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts