LM5000

SNVS176D – MAY 2004 – REVISED MARCH 2007

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BOOST REGULATOR OPERATION

The LM5000 utilizes a PWM control scheme to regulate the output voltage over all load conditions. The operation

can best be understood referring to the block diagram and Figure 21. At the start of each cycle, the oscillator

sets the driver logic and turns on the NMOS power device conducting current through the inductor, cycle 1 of

Figure 21 (a). During this cycle, the voltage at the COMP pin controls the peak inductor current. The COMP

voltage will increase with larger loads and decrease with smaller. This voltage is compared with the summation

of the SW volatge and the ramp compensation.The ramp compensation is used in PWM architectures to

eliminate the sub-harmonic oscillations that occur during duty cycles greater than 50%. Once the summation of

the ramp compensation and switch voltage equals the COMP voltage, the PWM comparator resets the driver

logic turning off the NMOS power device. The inductor current then flows through the output diode to the load

and output capacitor, cycle 2 of Figure 21 (b). The NMOS power device is then set by the oscillator at the end of

the period and current flows through the inductor once again.

The LM5000 has dedicated protection circuitry running during the normal operation to protect the IC. The

Thermal Shutdown circuitry turns off the NMOS power device when the die temperature reaches excessive

levels. The UVP comparator protects the NMOS power device during supply power startup and shutdown to

prevent operation at voltages less than the minimum input voltage. The OVP comparator is used to prevent the

output voltage from rising at no loads allowing full PWM operation over all load conditions. The LM5000 also

features a shutdown mode. An external capacitor sets the softstart time by limiting the error amp output range,

as the capacitor charges up via an internal 10µA current source.

The LM5000 is available in two operating frequency ranges. The LM5000-3 is pin selectable for either 300kHz

(FS Grounded) or 700kHz (FS Open). The LM5000-6 is pin selectable for either 600kHz (FS Grounded) or

1.3MHz (FS Open)

Operation

Figure 21. Simplified Boost Converter Diagram

(a) First Cycle of Operation (b) Second Cycle Of Operation

CONTINUOUS CONDUCTION MODE

The LM5000 is a current-mode, PWM regulator. When used as a boost regulator the input voltage is stepped up

to a higher output voltage. In continuous conduction mode (when the inductor current never reaches zero at

steady state), the boost regulator operates in two cycles.

In the first cycle of operation, shown in Figure 21 (a), the transistor is closed and the diode is reverse biased.

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