Datasheet LTC3703-5 (Analog Devices) - 10

LTC3703-5
U OPERATIO (Refer to Functional Diagram)
MOSFETs can occur. To prevent this from occuring, the cycle control set to 0%. As CSS continues to charge, the bottom driver return is brought out as a separate pin 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 supply output voltage to its regulated value, with no overshoot. is used on BGRTN, the switch node dV/dt could pull the The RUN/SS voltage will continue ramping until it reaches gate up 2V before the VGS of the bottom MOSFET has more an internal 4V clamp. Then the MIN feedback comparator than 0V across it. is enabled and the LTC3703-5 is in full operation. When the RUN/SS is low, the supply current is reduced to 25µA. DRVCC VIN LTC3703-5 DRV DB + CC V BOOST C OUT IN TG CB M1 0V NORMAL OPERATION SW SHUTDOWN START-UP CURRENT L LIMIT VOUT MIN COMPARATOR ENABLED 4V BG + OUTPUT VOLTAGE M2 COUT IN REGULATION BGRTN 3V RUN/SS SOFT-STARTS VRUN/SS OUTPUT VOLTAGE AND 37035 F02 0V TO –5V INDUCTOR CURRENT 1.4V 1V
Figure 2. Floating TG Driver Supply and Negative BG Return
MINIMUM 0V DUTY CYCLE
Constant Frequency
LTC3703-5 POWER ENABLE DOWN MODE 37035 F03 The internal oscillator can be programmed with an exter-
Figure 3. Soft-Start Operation in Start Up and Current Limit
nal resistor connected from fSET to ground to run between 100kHz and 600kHz, thereby optimizing component size, efficiency, and noise for the specific application. The
Current Limit
internal oscillator can also be synchronized to an external The LTC3703-5 includes an onboard current limit circuit clock applied to the MODE/SYNC pin and can lock to a that limits the maximum output current to a user-pro- frequency in the 100kHz to 600kHz range. When locked to grammed level. It works by sensing the voltage drop across an external clock, Pulse Skip Mode operation is automati- the bottom MOSFET and comparing that voltage to a user- cally disabled. Constant frequency operation brings with it programmed voltage at the IMAX pin. Since the bottom a number of benefits: Inductor and capacitor values can be MOSFET looks like a low value resistor during its on-time, chosen for a precise operating frequency and the feedback the voltage drop across it is proportional to the current loop can be similarly tightly specified. Noise generated by flowing in it. In a buck converter, the average current in the the circuit will always be at known frequencies. inductor is equal to the output current. This current also Subharmonic oscillation and slope compensation, com- flows through the bottom MOSFET during its on-time. mon headaches with constant frequency current mode Thus by watching the drain-to-source voltage when the switchers, are absent in voltage mode designs like the bottom MOSFET is on, the LTC3703-5 can monitor the LTC3703-5. output current. The LTC3703-5 senses this voltage and inverts it to allow it to compare the sensed voltage (which
Shutdown/Soft-Start
becomes more negative as peak current increases) with a The main control loop is shut down by pulling RUN/SS pin positive voltage at the IMAX pin. The IMAX pin includes a low. Releasing RUN/SS allows an internal 4µA current 12µA pull-up, enabling the user to set the voltage at IMAX source to charge the soft-start capacitor CSS. When CSS with a single resistor (RIMAX) to ground. See the Current reaches 1V, the main control loop is enabled with the duty Limit Programming section for RIMAX selection. 37035fa 10