Datasheet LTC7149 (Analog Devices) - 10

LTC7149 OPERATION The second limitation against full dropout operation is the To combat this issue, a separate LDO exists that is powered requirement for the BOOST to SW capacitor to refresh. from EXTVCC. As long as the input voltage is above 5V and When the top power MOSFET is on indefinitely during the EXTV – CC voltage is >3.2V above VOUT , this LDO will take dropout operation, the BOOST to SW capacitor slowly over and regulate the INTV – CC voltage to 3.1V above VOUT . gets depleted by the internal circuitry of the chip. When In applications where the output voltage is programmed the bottom switch does not turn on for at least 80ns for to –3.3V or below, it is recommended that the EXTVCC pin 8 periods, it is forced to turn on in order to guarantee be directly tied to the GND pin. Furthermore, if a separate sufficient voltage on the bootstrap capacitor. During a lower voltage rail exists on board that can supply INTVCC refresh, the bottom switch will only turn on for roughly current, then attaching that supply to EXTVCC will also 30% of the period to limit inductor ripple, thus limiting suffice provided that a 1µF ceramic bypass capacitor is output voltage ripple. placed from the EXTVCC pin to VOUT – physically close to the chip. Both examples should significantly reduce the
Input Voltage Regulation
power loss through the LDO. In certain applications, the input supply to the power regu- lator can exhibit fairly high output impedance. As a result,
VIN Undervoltage Programming
when the regulator is running at heavy loads, VIN might LTC7149 offers an accurate RUN threshold to start the droop more than desired. The input voltage regulation loop regulator. As a result, a resistor divider from VIN to GND allows the application to be programmed to decrease the can be placed with the intermediate node fed back to RUN peak inductor current level, and consequently the input to set an accurate VIN undervoltage threshold. As the input current draw, when it senses that the input voltage has voltage rises, the RUN voltage will increase above the VRUN dropped below a programmed threshold. If VINREG ever rising threshold (1.2V), and the regulator will turn on. falls below 2V above VOUT –, the regulator will decrease Similarly, once on, if the input voltage decreases below the the output current level in order to maintain the 2V at the VRUN falling threshold (1.1V), the regulator will turn off. pin. If this feature is not required, tie the VINREG pin to INTVCC to prevent this control loop from interfering with
VIN Overvoltage Protection
normal operation. In order to protect the internal power MOSFET devices
INTV
against transient voltage spikes, the LTC7149 constantly
CC Regulator
monitors the VIN pin for an overvoltage condition. When The LTC7149 has two onboard internal low dropout VIN + |VOUT –| rises above VOVLO the regulator suspends (LDO) regulators that power the drivers and internal bias operation by shutting off both power MOSFETs and dis- circuitry. Regardless of which one is in operation, the charges the ISET pin voltage to ground. Once VIN drops INTVCC must be bypassed to VOUT – with a minimum of below VOVLO – VOVLO(HYST), the regulator resumes normal 2.2µF ceramic capacitor. Good bypassing is necessary to switching operation. supply the high transient current required by the power MOSFET gate drivers.
Programming Switching Frequency
The first LDO is powered from VIN, and the INTVCC voltage is Connecting a resistor from the RT pin to SVOUT – programs regulated to 3.3V above VOUT –. The power dissipated across the switching frequency from 200kHz to 3MHz according this LDO would thus be equal to (VIN + |VOUT –|–3.3V)• to the following formula: IINTVCC. For a typical 1MHz application running in CCM, the current drawn from INTV 1011(1/ F) CC by the chip is roughly Frequency (Hz) = 20mA. Thus, if the input voltage is high, the power loss RT(Ω) and heat rise due to this LDO might be quite significant. Do not float the RT pin. 7149fa 10 For more information www.linear.com/LTC7149