Figure 10: Output Voltage Transient Response to a 14A load turn-off,

Power Supply Sequencing

The CS5166 offers inherent protection from undefined

start-up conditions, regardless of the 12V and 5V supply

power-up sequencing. The turn-on slew rates of the 12V

and 5V power supplies can be varied over wide ranges

without affecting the output voltage or causing detrimental

effects to the buck regulator.

Over-Current Protection

A loss-less hiccup mode current limit protection feature is

provided, requiring only the Soft Start capacitor to imple-

ment. The CS5166 provides overcurrent protection by sens-

ing the current through a “Droop” resistor, using an inter-

nal current sense comparator. The comparator compares

this voltage drop to an internal reference voltage of 76mV

(typical).

If the voltage drop across the “Droop” resistor exceeds this

threshold, the current sense comparator allows the fault

latch to be set. This causes the regulator to stop switching.

During this over current condition, the CS5166 stays off for

the time it takes the Soft Start capacitor to slowly discharge

by a 2µA current source until it reaches its lower 0.7V

threshold.

At that time the regulator attempts to restart normally by

delivering short gate pulses to both FETs. The CS5166 will

operate initially in its extended off time mode with a 50%

duty cycle, while the Soft Start capacitor is charged with a

60µA charge current. The gates will switch on while the

Soft Start capacitor is charged to its upper 2.7V threshold.

During an overload condition the Soft Start charge /dis-

charge current ratio sets the duty cycle for the pulses

(2µA/60µA = 3.3%), while actual duty cycle is half that due

than 1V. The Soft Start hiccup pulses last for a 3ms period

at the end of which the duty cycle repeats if a fault is

detected, otherwise normal operation resumes.

This protection scheme minimizes thermal stress to the reg-

ulator components, input power supply, and PC board

traces, as the over current condition persists. Upon

removal of the overload, the fault latch is cleared, allowing

normal operation to resume. The current limit trip point

can be adjusted through an external resistor, providing the

user with the current limit set-point flexibility.

Figure 11: Demonstration board hiccup mode short circuit protection.

Gate pulses are delivered while the Soft Start capacitor charges, and

cease during discharge.

Figure 12: Demonstration board Start up with regulator output shorted

to ground.

Overvoltage Protection

Overvoltage protection (OVP) is provided as result of the

no additional external components. The control loop

responds to an overvoltage condition within 100ns, causing

the top MOSFET to shut off, disconnecting the regulator

from its input voltage. The bottom MOSFET is then activat-

ed, resulting in a “crowbar” action to clamp the output

voltage and prevent damage to the load (see Figures 13

and 14). The regulator will remain in this state until the

overvoltage condition ceases or the input voltage is pulled

low. The bottom FET and board trace must be properly

designed to implement the OVP function. If a dedicated

OVP output is required, it can be implemented using the

circuit in Figure 15. In this figure the OVP signal will go

exceeds 20% of the voltage set by the particular DAC code

Trace 4 = 5V from PC Power Supply (2V/div.)

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

Trace 4 - 5V Supply Voltage (2V/div.)

Trace 3 - Soft Start Timing Capacitor (1V/div.)

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

Protection and Monitoring Features

Trace 1 - GATE(H) (10V/div)

Trace 2 - Inductor Switching Node (5V/div)

Trace 3 -Load Current (5A/div)

10

Application Information: continued