Datasheet

LM5088, LM5088-Q1
www.ti.com SNVS600H –DECEMBER 2008–REVISED MARCH 2013
DETAILED OPERATION
The LM5088 Wide Input Range Buck Controller features all the functions necessary to implement an efficient
high voltage step-down converter using a minimum number of external components. The control method is
based on peak current mode control utilizing an emulated current ramp. Peak current mode control provides
inherent line voltage 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 very small duty cycles necessary in high input voltage applications. The operating frequency is user
programmable from 50 kHz to 1 MHz. The LM5088-1 provides a ±5% frequency dithering function to reduce the
conducted and radiated EMI, while the LM5088-2 provides a versatile restart timer for overload protection.
Additional features include the low dropout bias regulator, tri-level enable input to control shutdown and standby
modes, soft-start, and voltage tracking and oscillator synchronization capability. The device is available in a
thermally enhanced HTSSOP-16 pin package.
See Block Diagram and Simplified Application Schematic. The LM5088 is well suited for a wide range of
applications where efficient step-down of high, unregulated input voltage is required. The LM5088’s typical
applications include Telecom, Industrial and Automotive.
High Voltage Low-Dropout Regulator
The LM5088 contains a high voltage, low-dropout regulator that provides the VCC bias supply for the controller
and the bootstrap MOSFET gate driver. The input pin (VIN) can be connected directly to an input voltage as high
as 75V. The output of the VCC regulator (7.8V) is internally current limited to 30 mA. Upon power up, the
regulator sources current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds
the upper VCC UV threshold of 4.0V and the EN pin is greater than 1.2 Volts, the output (HG) is enabled and a
soft-start sequence begins. The output is terminated if VCC falls below its lower UV threshold (3.8V) or the EN
pin falls below 1.1V. When VIN is less than VCC regulation point of 7.8V, then the internal pass device acts as a
switch. Thereby, VCC tracks VIN with a voltage drop determined by the R
DS(ON)
of the internal switch and
operating current of the controller. The required VCC capacitor value is dependant on system startup
characteristics with a minimum value no less than 0.1 µF.
An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary
voltage is greater than 8.2V, the internal regulator will be disabled. The VCC regulator series pass transistor
includes a diode between VCC and VIN that should not be forward biased in normal operation.
In high voltage applications, additional care should be taken to ensure that the VIN pin does not exceed the
absolute maximum voltage rating of 76V. During line or load transients, voltage ringing on the VIN pin that
exceeds the absolute maximum ratings may damage the IC. Both careful PC board layout and the use of high
quality bypass capacitors located close to the VIN and GND pins are essential.
Line Under-Voltage Detector
The LM5088 contains a dual level under-voltage lockout (UVLO) circuit. When the EN pin is below 0.4V, the
controller is in a low current shutdown mode. When the EN pin is greater than 0.4V but less than 1.2V, the
controller is in a standby mode. In standby mode the VCC regulator is active but the output switch is disabled
and the SS pin is held low. When the EN pin exceeds 1.2V and VCC exceeds the VCC UV threshold, the SS pin
and the output switch is enabled and normal operation begins. An internal 5 µA pull-up current source at the EN
pin configures the controller to be fully operational if the EN pin is left open.
An external VIN UVLO set-point voltage divider from VIN to GND can be used to set the minimum startup input
voltage of the controller. The divider must be designed such that the voltage at the EN pin exceeds 1.2V (typ)
when VIN is in the desired operating range. The internal 5 µA pull-up current source must be included in
calculations of the external set-point divider. 100 mV of hysteresis is included for both the shutdown and standby
thresholds. The EN pin is internally connected to a 1 kΩ resistor and an 8V zener clamp. If the voltage at the EN
pin exceeds 8V, the bias current for the EN pin will increase at the rate of 1mA/V. The voltage at the EN pin
should never exceed 14V.
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