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AMS4106

5A SYNCHRONUS PWM BUCK CONVERTER

Advanced Monolithic Systems Inc 

AMS4106

DETAIL DESCRIPTION and GUIDELINES

Start up from enable

When the enable is low the part is completely shut down with the nano-amps only of leakage current. When the enable is taken

above the turn on threshold it powers up part. There are 2 soft start mechanisms in operation during start up, error amplifier and

external. For small output capacitance (ceramic only solutions), the compensation can be used for soft start, and the soft start pin

is left open. For situation requiring slower soft start or where large output capacitors are used a separate soft start pin is used.

This charges the external capacitor with around 2.2 µA current allowing small ceramic capacitors to be used. When Power good

senses the output is almost at its final value the error amplifier current is turned on to its normal running current overriding the

startup current.

Enable connected to Vin

The part initiates its soft start described above at UVLO threshold of around 4.75V.

PWM frequency

The default it internally set to 250 kHz with the FSET pin left open. Adding a resistor to ground switches it into the external set

mode. A 68K resistor to ground gives approximately 250 kHz PWM frequency. Care should be taken to keep the resistor close

to the part as pick-up on this pin can cause jitter.

Over-current shutdown

If over-current is sensed the part shuts down and initiates the soft start sequence providing a hiccup function. This means that

shorting the output is non destructive and will run a low supply current. When the output shuts down the low-side FET is turned

off giving a tri-state output. This helps prevent negative output voltages being generated in an overload condition where the load

significantly reduces (due to system reset etc) as a result of the output voltage failing. Power good is held low during over

current.

Synchronous operation

With an external FET fitted between Phase and ground the parts enters synchronous operation. Gate time is adaptively

controlled allowing large freedom in the choice of output FET. For highest performance the lowest gate charge FET typically

will give the best overall efficiency. The gate drive features a medium drive capability of around 0.5-1A removing the need

series gate resistors for most applications. Due to the fast switching on the phase node it is important that the FET is placed very

close to the part with very short paths for both ground and the phase node. Large parasitic inductance can cause large negative

spikes on the switch output causing jitter and in severe circumstances potential circuit malfunction.

Asynchronous operation

A Schotty diode can be used in place of the FET for certain applications, no other changes are required to accommodate this

mode. The gate drive pin should be left open. This is at the expense of full load efficiency especially at low output voltage.

Transient performance is also reduced.

For applications when light load higher efficiency is required Asynchronous operation is preferred.

For Applications requiring HOT switching Asynchronous operation is preferred preventing unwanted dips on the output supply.

Duty cycle considerations

For low input output ratios greater than 50% duty cycle the maximum output should be de-rated to reduce package heating and

thermal shutdown.

For high input output ratios the maximum frequency is determined by the minimum useable duty cycle, for this part it is around

120ns, shorter duty cycles could cause jitter or pulse skipping. For a 0.8v output and a switching frequency of 500kHz a

maximum input voltage of around 14v can be accommodated at light load rising to about 20v at 4A.

Bootstrap Circuit

To allow operation over a very large range the devices uses an internal boost regulator and internal boost diode. The boost

capacitor supplies the output bias current requirements. The regulator is set to the minimum voltage required to give operation at

full output current. It is important that the capacitor is large enough to supply the current for the full on time for large duty 1µF

is recommended for short duty cycle<10% 100nf is suitable. Using a 1µF boost capacitor for all applications has no detrimental

effect. The voltage across the capacitor is small (around 3v) so small ceramic case sizes can be used.

Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723

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