Theory of Operation

Basic Operation of the Current-Mode Controller

The output voltage is held at a constant value by the main

control loop, which is made up of the

error amplifier, the cur-

rent sense amplifier, and the PWM comparator (refer to the

Block Diagram,

Figure 3).

The LM2641 controller has two primary modes of operation:

Forced Pulse-Width Modulation (FPWM) where the control-

ler always operates at a fixed frequency, and

Pulse-Skipping

mode where the controller frequency decreases at reduced

output loads to improve light-load efficiency.

FPWM Mode of Operation

Pulling the FPWM pin low initiates a mode of operation

called Forced Pulse-Width Modulation (FPWM). This means

that the LM2641 will

always operate at a fixed frequency, re-

gardless of output load. The cycle of operation is:

The high-side FET switch turns ON at the beginning of every

clock cycle, causing current to flow through the inductor. The

inductor current ramps up, causing a voltage drop across the

sense resistor, and this voltage is amplified by the current

sense amplifier.

The voltage signal from the current sense amplifier is applied

to the input of the PWM comparator, where it is compared to

the control level set by the error amplifier. Once the current

sense signal reaches this control voltage, the PWM com-

parator resets the driver logic which turns OFF the high-side

FET switch.

The low-side FET switch turns on after a delay time which is

the lesser of either:

(a) The time it takes the SW pin voltage to reach zero (this

voltage is sensed by the shoot-through protection circuitry).

(b) 100 ns, which is the pre-set value for maximum delay.

When operating at very light loads (in FPWM mode), the in-

ductor current must flow in a negative direction through the

low-side FET switch in order to maintain the fixed-frequency

mode of operation. For this reason, the built-in zero cross

detector is disabled when ever FPWM mode is activated

(that is, when ever the FPWM pin is pulled to a low state).

It should be noted that if the FPWM pin is high (operation de-

scribed in next section), the zero cross detector will turn OFF

the low-side FET switch anytime the inductor current drops

to zero (which prevents negative inductor current).

Pulse-Skipping Mode of Operation

Pulling the FPWM pin high allows the LM2641 to operate in

pulse-skipping mode at light loads, where the switching fre-

quency decreases as the output load is reduced. The con-

troller will operate in fixed-frequency mode, as described in

the previous section, if the output load current is sufficiently

high.

Pulse-skipping results in higher efficiency at light loads, as

decreasing the switching frequency reduces switching

Normalized Error Amplifier Voltage

vs

Junction Temperature

DS100949-26

Error Amplifier Gm vs

Junction Temperature

DS100949-27

Normalized Oscillator Frequency

vs

Junction Temperature

DS100949-28

Shutdown Quiescent Current

And Standby Quiescent Current vs

Supply Voltage

DS100949-29

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