RT = 13.1 x 10

9

x

1

- 83 ns

LM5001, LM5001-Q1

www.ti.com

SNVS484H – JANUARY 2007 – REVISED JULY 2015

7.3 Feature Description

7.3.1 High-Voltage VCC Regulator

The LM5001 VCC Low Drop Out (LDO) regulator allows the LM5001 to operate at the lowest possible input

voltage. The VCC pin voltage is very nearly equal to the input voltage from 2.8 V up to approximately 6.9 V. As

the input voltage continues to increase, the VCC pin voltage is regulated at the 6.9 V set-point. The total input

operating range of the VCC LDO regulator is 3.1 V to 75 V.

The output of the VCC regulator is current limited to 20 mA. During power up, the VCC regulator supplies current

into the required decoupling capacitor (0.47 µF or greater ceramic capacitor) at the VCC pin. When the voltage at

the VCC pin exceeds the VCC UVLO threshold of 2.8 V and the EN pin is greater than 1.26 V the PWM

controller is enabled and switching begins. The controller remains enabled until VCC falls below 2.7 V or the EN

pin falls below 1.16 V.

An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary

voltage is greater than 6.9 V, the internal regulator essentially shuts off, and internal power dissipation decreases

by the VIN voltage times the operating current. The overall converter efficiency improves if the VIN voltage is

much higher than the auxiliary voltage. The externally applied VCC voltage should not exceed 14 V. The VCC

regulator series pass MOSFET includes a body diode (Functional Block Diagram ) between VCC and VIN that

should not be forward biased in normal operation. Therefore, the auxiliary VCC voltage should never exceed the

VIN voltage.

In high voltage applications extra care should be taken to ensure the VIN pin does not exceed the absolute

maximum voltage rating of 76 V. Voltage ringing on the VIN line during line transients that exceeds the Absolute

Maximum Ratings damages the IC. Both careful PC board layout and the use of quality bypass capacitors

located close to the VIN and GND pins are essential.

7.3.2 Oscillator

A single external resistor connected between RT and GND pins sets the LM5001 oscillator frequency. To set a

(1)

The tolerance of the external resistor and the frequency tolerance indicated in the Electrical Characteristics must

be taken into account when determining the worst case frequency range.

7.3.3 External Synchronization

The LM5001 can be synchronized to the rising edge of an external clock. The external clock must have a higher

frequency than the free running oscillator frequency set by the RT resistor. The clock signal should be coupled

through a 100 pF capacitor into the RT pin. A peak voltage level greater than 2.6 V at the RT pin is required for

detection of the sync pulse. The DC voltage across the RT resistor is internally regulated at 1.5 V. The negative

portion of the AC voltage of the synchronizing clock is clamped to this 1.5 V by an amplifier inside the LM5001

with ~100

Ω output impedance. Therefore, the AC pulse superimposed on the RT resistor must have positive

pulse amplitude of 1.1 V or greater to successfully synchronize the oscillator. The sync pulse width measured at

the RT pin should have a duration greater than 15 ns and less than 5% of the switching period. The sync pulse

rising edge initiates the internal CLK signal rising edge, which turns off the power MOSFET. The RT resistor is

always required, whether the oscillator is free running or externally synchronized. The RT resistor should be

located very close to the device and connected directly to the RT and GND pins of the LM5001.

7.3.4 Enable / Standby

The LM5001 contains a dual level Enable circuit. When the EN pin voltage is below 450 mV, the IC is in a low

current shutdown mode with the VCC LDO disabled. When the EN pin voltage is raised above the shutdown

threshold but below the 1.26 V standby threshold, the VCC LDO regulator is enabled, while the remainder of the

IC is disabled. When the EN pin voltage is raised above the 1.26 V standby threshold, all functions are enabled

and normal operation begins. An internal 6 µA current source pulls up the EN pin to activate the IC when the EN

pin is left disconnected.

Copyright © 2007–2015, Texas Instruments Incorporated

Submit Documentation Feedback

9

Product Folder Links: LM5001 LM5001-Q1