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LM25119 Datenblatt(PDF) 11 Page - Texas Instruments |
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LM25119 Datenblatt(HTML) 11 Page - Texas Instruments |
11 / 33 page L COUT SW VCC VOUT LM25119/25119Q www.ti.com SNVS680E – AUGUST 2010 – REVISED FEBRUARY 2013 Detailed Operating Description The LM25119 high voltage switching regulator features all of the functions necessary to implement an efficient dual channel buck regulator that operates over a very wide input voltage range. The LM25119 may be configured as two independent regulators or as a single high current regulator with two interleaved channels. This easy to use regulator integrates high-side and low-side MOSFET drivers capable of supplying peak currents of 2.5 Amps (VCC=8V). The regulator control method is based on current mode control utilizing an emulated current ramp. Emulated peak current mode control provides inherent line feed-forward, cycle-by-cycle current limiting and ease of loop compensation. The use of an emulated control ramp reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable processing of the very small duty cycles necessary in high input voltage applications. The switching frequency is user programmable from 50kHz to 750kHz. An oscillator/synchronization pin allows the operating frequency to be set by a single resistor or synchronized to an external clock. An under- voltage lockout and channel2 enable pin allows either both regulators to be disabled or channel2 to be disabled with full operation of channel1. Fault protection features include current limiting, thermal shutdown and remote shutdown capability. The under-voltage lockout input enables both channels when the input voltage reaches a user selected threshold and provides a very low quiescent shutdown current when pulled low. The LLP32 package features an exposed pad to aid in thermal dissipation. High Voltage Start-Up Regulator The LM25119 contains two internal high voltage bias regulators, VCC1 and VCC2, that provide the bias supply for the PWM controllers and gate drive for the MOSFETs of each regulator channel. The input pin (VIN) can be connected directly to an input voltage source as high as 42 volts. The outputs of the VCC regulators are set to 7.6V. When the input voltage is below the VCC set-point level, the VCC output will track the VIN with a small dropout voltage. If VCC1 is in an under voltage condition, channel2 will be disabled. This interdependence is necessary to prevent channel2 from running open loop in the single output interleaved mode when the channel2 error amplifier is disabled (if either VCC is in UV, both channels are disabled). The outputs of the VCC regulators are current limited at 25mA (minimum) output capability. Upon power-up, the regulators source current into the capacitors connected to the VCC pins. When the voltage at the VCC pins exceed 4.0V and the UVLO pin is greater than 1.25V, both channels are enabled and a soft-start sequence begins. Both channels remain enabled until either VCC pin falls below 3.8V, the UVLO pin falls below 1.25V or the die temperature exceeds the thermal limit threshold. When operating at higher input voltages the bias power dissipation within the controller can be excessive. An output voltage derived bias supply can be applied to a VCC pins to reduce the IC power dissipation. The VCCDIS input can be used to disable the internal VCC regulators when external biasing is supplied. If VCCDIS >1.25V, the internal VCC regulators are disabled. The externally supplied bias should be coupled to the VCC pins through a diode, preferably a Schottky (low forward voltage). VCCDIS has a 500k Ω internal pull-down resistance to ground for normal operation with no external bias. The internal pull-down resistance can be overridden by pulling VCCDIS above 1.25V through a resistor divider connected to an external bias supply. The VCC regulator series pass transistor includes a diode between VCC and VIN that should not be forward biased in normal operation. If the external bias winding can supply VCC greater than VIN, an external blocking diode is required from the input power supply to the VIN pin to prevent the external bias supply from passing current to the input supply through the VCC pins. For VOUT between 5V and 14.5V, VOUT can be connected directly to VCC through a diode. For VOUT < 5V, a bias winding on the output inductor can be added as shown in Figure 3. Figure 3. VCC Bias Supply with Additional Inductor Winding Copyright © 2010–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM25119/25119Q |
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