CS5307GDWR24 datasheet(16/24 Pages) ONSEMI

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CS5307

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Table 1. Fault Protection Logic

Fault Modes

Stop Switching

SS Pin Characteristics

Reset Method

Undervoltage Lockout

Yes

−5.0 μA

SS < 0.3 V

VID−11111

Yes

−5.0 μA

Change VID Code

Phase Over Current

(0.33 V Limit)

Not Affected

Automatic

The latest VRM and processor specifications require a

power supply to turn its output off in the event of a 11111

VID code. When the DAC sees such a code, the GATE pins

stop switching and go low. This condition is described in

Table 1.

Design Procedure

1. Output Capacitor Selection

The output capacitors filter the current from the output

inductor and provide a low impedance for transient load

current changes. Typically, microprocessor applications

require both bulk (electrolytic, tantalum) and low

impedance, high frequency (ceramic) types of capacitors.

The bulk capacitors provide “hold up” during transient

loading. The low impedance capacitors reduce steady−state

ripple and bypass the bulk capacitance when the output

current changes very quickly. The microprocessor

manufacturers usually specify a minimum number of

ceramic capacitors. The designer must determine the

number of bulk capacitors.

Choose the number of bulk output capacitors to meet the

peak transient requirements. The formula below can be used

to provide a starting point for the minimum number of bulk

capacitors (NOUT,MIN):

NOUT,MIN + ESR per capacitor @

DIO,MAX

DVO,MAX

(1)

In reality, both the ESR and ESL of the bulk capacitors

determine the voltage change during a load transient

according to:

DVO,MAX + (DIO,MAX Dt) @ ESL ) DIO,MAX @ ESR (2)

Unfortunately, capacitor manufacturers do not specify the

ESL of their components and the inductance added by the

PCB traces is highly dependent on the layout and routing.

Therefore, it is necessary to start a design with slightly more

than the minimum number of bulk capacitors and perform

transient testing or careful modeling/simulation to

determine the final number of bulk capacitors.

2. Output Inductor Selection

The output inductor may be the most critical component

in the converter because it will directly effect the choice of

other components and dictate both the steady−state and

transient performance of the converter. When selecting an

inductor, the designer must consider factors such as DC

current, peak current, output voltage ripple, core material,

magnetic saturation, temperature, physical size and cost

(usually the primary concern).

In general, the output inductance value should be

elecrrically and physically as small as possible to provide the

best transient response at minimum cost. If a large

inductance value is used, the converter will not respond

quickly to rapid changes in the load current. On the other

hand, too low an inductance value will result in very large

ripple currents in the power components (MOSFETs,

capacitors, etc.) resulting in increased dissipation and lower

converter efficiency. Increased ripple currents force the

designer to use higher rated MOSFETs, oversize the thermal

solution, and use more, higher rated input and output

capacitors, adversely affecting converter cost.

One method of calculating an output inductor value is to

size the inductor to produce a specified maximum ripple

current in the inductor. Lower ripple currents will result in

less core and MOSFET losses and higher converter

efficiency. Equation 3 may be used to calculate the

minimum inductor value to produce a given maximum

ripple current (α) per phase. The inductor value calculated

by this equation is a minimum because values less than this

will produce more ripple current than desired. Conversely,

higher inductor values will result in less than the selected

maximum ripple current.

LoMIN +

(VIN * VOUT) @ VOUT

(a @ IO,MAX @ VIN @ fSW)

(3)

α is the ripple current as a percentage of the maximum

output current per phase (α = 0.15 for ±15%, α = 0.25 for

±25%, etc.). If the minimum inductor value is used, the

inductor current will swing ± α% about its value at the

center. Therefore, for a four−phase converter, the inductor

must be designed or selected such that it will not saturate

with a peak current of (1 + α) ⋅ IO,MAX/4.

The maximum inductor value is limited by the transient

response of the converter. If the converter is to have a fast

transient response, the inductor should be made as small as

possible. If the inductor is too large its current will change

too slowly, the output voltage will droop excessively, more

bulk capacitors will be required and the converter cost will

be increased. For a given inductor value, it is useful to

determine the times required to increase or decrease the

current.

For increasing current:

DtINC + Lo @ DIO (VIN * VOUT)

(3.1)

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