UCC2541

SLUS621A − AUGUST 2004 − SEPTEMBER 2005

11

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APPLICATION INFORMATION

SYNCIN

SS

RAMP

G2C

G1

G2

0.5 V

3.3 V

ILOAD

VLOAD

UDG−04046

Figure 4. Typical Hiccup Mode waveforms

COMP, VEA− and CEA− pins: Voltage and Current Error Amplifiers

From no-load to full rated load operating conditions, the UCC2541 operates as a voltage mode controller. Above

the programmed rated current, there are two levels of over current protection; constant current limit and

overcurrent reset/retry. This section gives suggestions on how to design the voltage controller and current

controller so that they interact with one another in a stable fashion. Refer to the functional diagram of the voltage

and current error amplifiers in Figure 3. The voltage error amplifier in the figure shows three non-inverting inputs.

The lowest of the three non-inverting inputs (1.5 V, SS and TR) is summed with the inverting input to achieve

the voltage error signal. The lowest of the two outputs drives the inverting stage which in turn, drives the

modulator.

During steady state voltage control operation, the feedback elements in the current loop have no effect on the

loop stability. When current limit occurs, the voltage error amplifier effectively shuts OFF and the current error

amplifier takes control. During steady state current limit operation, the negative feedback elements in the

voltage error amplifier loop become positive feedback elements in the current error amplifier loop. In order for

the current error amplifier to be stable, the impedances in the feedback path of the current error amplifier must

be lower than the impedances in the feedback path of the voltage error amplifier. This means that resistors in

the current error amplifier negative feedback path must be less than the resistors in the voltage error amplifier

negative feedback path. Also capacitors in the current error amplifier negative feedback path must be larger

than capacitors in the negative feedback path of the voltage error amplifier negative feedback path.

(Capacitance is really an admittance value rather than an impedance value). This concept is illustrated in

Figure 3.