Block Diagram

20098303

Boost Regulator Operation

The LM5000EP 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 3. 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 3 (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 com-

pensation.The ramp compensation is used in PWM architec-

tures 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 3 (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 LM5000EP 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

LM5000EP 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 LM5000EP is available in two operating frequency

ranges. The LM5000-3EP is pin selectable for either 300kHz

(FS Grounded) or 700kHz (FS Open). The LM5000-6EP is

pin selectable for either 600kHz (FS Grounded) or 1.3MHz

(FS Open)

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