LM5088

SNVS600G – DECEMBER 2008 – REVISED MARCH 2011

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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

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 k

Ω 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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