6

Applications Information: continued

ing of the output voltage, since the noise associated with

long feedback traces can be effectively filtered.

Line and load regulation are drastically improved because

there are two independent voltage loops. A voltage mode

controller relies on a change in the error signal to compen-

sate for a deviation 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 sig-

maintains a fixed error signal for both line and load varia-

tion, since the ramp signal is affected by both line and load.

Constant Off Time

To maximize transient response, the CS5151 uses a con-

stant off time method to control the rate of output pulses.

During normal operation, the off time of the high side

varies switch on time. The PWM comparator monitors the

output voltage ramp, and terminates the switch on time.

Constant off time provides a number of advantages. Switch

duty cycle can be adjusted from 0 to 100% on a pulse by

pulse basis when responding to transient conditions. Both

0% and 100% duty cycle operation can be maintained for

extended periods of time in response to load or line tran-

sients. PWM slope compensation to avoid sub-harmonic

oscillations at high duty cycles is avoided.

Switch on time is limited by an internal 30µs timer, mini-

mizing stress to the power components.

Programmable Output

The CS5151 is designed to provide two methods for pro-

gramming the output voltage of the power supply. A four

bit on board digital to analog converter (DAC) is used to

program the output voltage from 2.14V to 3.54V in 100mV

steps, depending on the digital input code. If all four bits

are left open, the CS5151 enters adjust mode. In adjust

mode, the designer can choose any output voltage by using

ditional controllers. The CS5151 is specifically designed to

be upwards compatible with the CS5156, which uses a five

bit DAC code.

Start Up

monitor threshold, the soft start and gate pins are held low.

The FAULT latch is reset (no Fault condition). The output

of the error amplifier (COMP) is pulled up to 1V by the

threshold, the Gate output is activated, and the soft start

capacitor begins charging. The Gate output will remain on,

enabling the NFET switch, until terminated by either the

PWM comparator, or the maximum on time timer.

If the maximum on time is exceeded before the regulator

output voltage achieves the 1V level, the pulse is terminat-

ed. The Gate pin drives low for the duration of the extend-

ed off time. This time is set by the time out timer and is

approximately equal to the maximum on time, resulting in

a 50% duty cycle. Then, the Gate pin will drive high, and

the cycle repeats.

When regulator output voltage achieves the 1V level pre-

sent at the COMP pin, regulation has been achieved and

normal off time will ensue. The PWM comparator termi-

cycle as required to ensure the regulator output voltage

tracks the output of the error amplifier.

The soft start and COMP capacitors will charge to their

final levels, providing a controlled turn on of the regulator

output. Regulator turn on time is determined by the COMP

capacitor charging to its final value. Its voltage is limited

by the soft start COMP clamp and the voltage on the soft

start pin (see Figures 2 and 3).

Figure 2: CS5151 demonstration board startup in response to increasing

12V and 5V input voltages. Extended off time is followed by normal off

time operation when output voltage achieves regulation to the error

amplifier output.

Figure 3: CS5151 demonstration board startup waveforms.

Trace 1 - Regulator Output Voltage (1V/div.)

Trace 3 - COMP Pin (error amplifier output) (1V/div.)

Trace 4 - Soft Start Pin (2V/div.)

Trace 1 - Regulator Output Voltage (1V/div.)

Trace 2 - Inductor Switching Node (2V/div.)

Trace 4 - 5V Input (1V/div.)