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LTM4600EVPBF Datasheet(PDF) 13 Page - Linear Technology |
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LTM4600EVPBF Datasheet(HTML) 13 Page - Linear Technology |
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13 / 24 page  LTM4600 13 4600fa Enable The RUN/SS pin can be driven from logic as shown in Figure 5. This function allows the LTM4600 to be turned on or off remotely. The ON signal can also control the sequence of the output voltage. Figure 5. Enable Circuit with External Logic RUN/SS 4600 F05 LTM4600 PGND 2N7002 SGND ON Figure 6. Output Voltage Tracking with the LTC2923 Controller Q1 VCC VIN VIN RONB VIN 5V RTB1 RTB2 49.9k 1.8V 3.3V RTA2 RTA1 RONA ON RAMPBUF TRACK1 TRACK2 FB1 GATE LTC2923 GND 4600 F06 RAMP 66.5k 1.5V LTM4600 VIN VOUT LTM4600 DC/DC VIN VOUT VOSET VOSET FB2 SDO STATUS Output Voltage Tracking For the applications that require output voltage tracking, several LTM4600 modules can be programmed by the power supply tracking controller such as the LTC2923. Figure 6 shows a typical schematic with LTC2923. Coin- APPLICATIO S I FOR ATIO cident, ratiometric and offset tracking for VO rising and falling can be implemented with different sets of resistor values. See the LTC2923 data sheet for more details. EXTVCC Connection An internal low dropout regulator produces an internal 5V supply that powers the control circuitry and FET drivers. Therefore, if the system does not have a 5V power rail, the LTM4600 can be directly powered by VIN. The gate driver current through LDO is about 18mA. The internal LDO power dissipation can be calculated as: PLDO_LOSS = 18mA • (VIN – 5V) The LTM4600 also provides an external gate driver volt- age pin EXTVCC. If there is a 5V rail in the system, it is recommended to connect EXTVCC pin to the external 5V rail. Whenever the EXTVCC pin is above 4.7V, the internal 5V LDO is shut off and an internal 50mA P-channel switch connects the EXTVCC to internal 5V. Internal 5V is supplied from EXTVCC until this pin drops below 4.5V. Do not apply more than 6V to the EXTVCC pin and ensure that EXTVCC < VIN. The following list summaries the possible connec- tions for EXTVCC: 1. EXTVCC grounded. Internal 5V LDO is always powered from the internal 5V regulator. 2. EXTVCC connected to an external supply. Internal LDO is shut off. A high efficiency supply compatible with the MOSFET gate drive requirements (typically 5V) can im- prove overall efficiency. With this connection, it is always required that the EXTVCC voltage can not be higher than VIN pin voltage. Discontinuous Operation and FCB Pin The FCB pin determines whether the internal bottom MOSFET remains on when the inductor current reverses. There is an internal 4.75k pulling down resistor connecting this pin to ground. The default light load operation mode is forced continuous (PWM) current mode. This mode provides minimum output voltage ripple. |
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