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 CS5150H 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 CS5150H 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 CS5150H enters adjust mode. In adjust

mode, the designer can choose any output voltage by using

ditional controllers. The CS5150H is specifically designed

to be upwards compatible with the CS5155H, 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 GateH output is activated, and the soft start

capacitor begins charging. The GateH 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 GateH pin drives low, and the GateL pin drives

high for the duration of the extended 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. The

GateL pin will then drive low, the GateH 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: CS5150H demonstration board startup in response to increas-

ing 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: CS5150H 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.)

Applications Information: continued

6