Datasheet

L
C
OUT
SW
VCC
V
OUT
LM25119/25119Q
www.ti.com SNVS680G –AUGUST 2010–REVISED JANUARY 2014
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 = 8 V). 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 50 kHz to 750 kHz. 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 WQFN-32 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 V. The outputs of the VCC regulators are set to 7.6
V. 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 25 mA (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 4.0 V and the UVLO pin is greater than 1.25 V, both channels are enabled and a soft-start sequence
begins. Both channels remain enabled until either VCC pin falls below 3.8 V, the UVLO pin falls below 1.25 V 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
>1.25 V, 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 500 kΩ 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 1.25 V 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 5 V and 14.5 V, VOUT can be connected directly to VCC through a
diode. For VOUT < 5 V, 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 © 2010–2014, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LM25119/25119Q