eorex

EP4001

Aug. 2006

www.eorex.com

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Low Noise Fixed Frequency Operation Oscillator

The frequency of operation is internally set to 450KHz.

Error Amp:

The internal 1.22V reference voltage is compared to the voltage at the FB pin to generate an

according to the equation:

⎟

⎠

⎞

⎜

⎝

⎛

+

R2

R1

1

Where FB=1.22V

Due to device performance limitation, the EP4001 is limited to applications with output voltage less than

4.2V, until the problem is corrected at later date.

PWM Mode

Refer to Figure 3, functional block diagram. The main gain block is a comparator that sums four signals:

feedback voltage, reference, current-sensing and slope compensation ramp. This direct-summing

method approaches the ideal of cycle-by-cycle control of output voltage. Under heavy loads, the

controller operates in full PWM mode. Every pulse from the oscillator sets the output latch and turns on

the NMOS switch for a period determined by the duty factor and current limit.

PFM Mode Operation

Portable devices frequently spend extended time in low power or standby mode, only switching to high

power mode when specific functions are enabled. Maintain high efficiency over a wide range of load

current is critical for prolong battery life. In addition to its high efficiency at moderate and heavy loads,

the EP4001 includes automatic PFM operation that improves efficiency of the converter at light load.

PFM operation is initiated if the output load current falls below an internally programmed threshold. Once

initiated, the PFM operation circuitry shuts down most of the device, only keeping alive the circuitry

required to monitor the output voltage.