NCV8800 Series

http://onsemi.com

9

APPLICATIONS INFORMATION

NCV8800

Switch

FB2

Power Up/Down

Sequence and

ENABLE

−

+

R2

21.4 k

Error Amp

1.20 V

56

µA

FB1

R1*

Figure 7.

*The value of R1

is dependent

on the output

voltage option

and is between

25 k and 200 k.

Increasing the Output Voltage

Adjustments to the output voltage can be made with an

typically be 56

be given to the tracking feature and temperature coefficient

and matching of internal and external resistors. Output

tracking always follows the Feedback pins (FB1 and FB2).

The typical temperature coefficient for R1 and R2 is

+4600 ppm/

°C.

THEORY OF OPERATION

generated by the ESR of the output capacitors. This ramp is

proportional to the AC current through the main inductor

and is offset by the value of the DC output voltage. This

control scheme inherently compensates for variations in

either line or load conditions, since the ramp signal is

generated from the output voltage itself. This control

scheme differs from traditional techniques such as voltage

mode, which generates an artificial ramp, and current mode,

which generates a ramp from inductor current.

Ramp Signal

Error Signal

Error Amplifier

COMP

GATE(L)

GATE(H)

Output

Voltage

Feedback

PWM Comparator

Reference

Voltage

voltage is used to generate both the error signal and the ramp

signal. Since the ramp signal is simply the output voltage, it

is affected by any change in the output regardless of the origin

of the change. The ramp signal also contains the DC portion

of the output voltage, which allows the control circuit to drive

the main switch to 0% or 100% duty cycle as required.

A change in line voltage changes the current ramp in the

control scheme to compensate the duty cycle. Since the

change in the inductor current modifies the ramp signal, as

advantages in line transient response.

A change in load current will have an effect on the output

voltage, altering the ramp signal. A load step immediately

changes the state of the comparator output, which controls

the main switch. Load transient response is determined only

by the comparator response time and the transition speed of

the main switch. The reaction time to an output load step has

no relation to the crossover frequency of the error signal

loop, as in traditional control methods.

The error signal loop can have a low crossover frequency,

since transient response is handled by the ramp signal loop.

The main purpose of this “slow” feedback loop is to provide

DC accuracy. Noise immunity is significantly improved,

since the error amplifier bandwidth can be rolled off at a low

frequency. Enhanced noise immunity improves remote

sensing of the output voltage, since the noise associated with

long feedback traces can be effectively filtered.

Line and load regulations are drastically improved

because there are two independent voltage loops. A voltage

mode controller relies on a change in the error signal to

compensate for a derivation in either line or load voltage.

This change in the error signal causes the output voltage to

change corresponding to the gain of the error amplifier,

which is normally specified as line and load regulation. A

current mode controller maintains fixed error signal under

deviation in the line voltage, since the slope of the ramp

signal changes, but still relies on a change in the error signal

a fixed error signal for both line and load variations, since

both line and load affect the ramp signal.

Constant Frequency Operation

During normal operation, the oscillator generates a 200 kHz,

90% duty cycle waveform. The rising edge of this waveform

determines the beginning of each switching cycle, at which

point the high−side switch will be turned on. The high−side

switch will be turned off when the ramp signal intersects the

output of the error amplifier (COMP pin voltage).

Therefore, the switch duty cycle can be modified to regulate

the output voltage to the desired value as line and load

conditions change.