MAX1540A Datasheet, PDF(37/49 Page) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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MAX1540A Datasheet, PDF (37/49 Pages) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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Dual Step-Down Controllers with Saturation

Protection, Dynamic Output, and Linear Regulator

For high-power applications that do not require high-

accuracy current sensing or inductor-saturation protec-

tion, the MAX1540A/MAX1541 can use the low-side

MOSFETâs on-resistance as the current-sense element

(RSENSE = RDS(ON)) by connecting CSN_ to the drain

of NL_ and CSP_ to the source of NL_ (Figure 14c). Use

the worst-case maximum value for RDS(ON) from the

MOSFET data sheet, and add some margin for the rise

in RDS(ON) with temperature. A good general rule is to

allow 0.5% additional resistance for each Â°C of temper-

ature rise. Inductor-saturation protection must be dis-

abled with this configuration (LSAT = GND) since the

inductor current is only properly sensed when the low-

side MOSFET is turned on.

Alternatively, high-power applications that require

inductor saturation can constantly detect the inductor

current by connecting a series RC circuit across the

inductor (Figure 14d) with an equivalent time constant:

= CEQ

REQ

where RL is the inductorâs series DC resistance. In this

configuration, the current-sense resistance is equiva-

lent to the inductorâs DC resistance (RSENSE = RL). Use

the worst-case inductance and RL values provided by

the inductor manufacturer, adding some margin for the

inductance drop over temperature and load.

In all cases, ensure an acceptable valley current-limit

threshold voltage and inductor-saturation configura-

tions despite inaccuracies in sense-resistance values.

Output Capacitor Selection

The output filter capacitor must have low enough equiv-

alent series resistance (ESR) to meet output ripple and

load-transient requirements, yet have high enough ESR

to satisfy stability requirements.

Table 9. Current-Sense Configurations

For processor-core voltage converters and other appli-

cations where the output is subject to violent load tran-

sients, the output capacitorâs size depends on how

much ESR is needed to prevent the output from dipping

too low under a load transient. Ignoring the sag due to

finite capacitance:

RESR

â¤

VSTEP

ÎILOAD(MAX)

In applications without large and fast load transients,

the output capacitorâs size often depends on how much

ESR is needed to maintain an acceptable level of out-

put voltage ripple. The output ripple voltage of a step-

down controller equals the total inductor ripple current

multiplied by the output capacitorâs ESR. Therefore, the

maximum ESR required to meet ripple specifications is:

RESR

â¤

VRIPPLE

ÎILOAD(MAX)

Ã LIR

The actual capacitance value required relates to the

physical size needed to achieve low ESR, as well as to

the chemistry of the capacitor technology. Thus, the

capacitor is usually selected by ESR and voltage rating

rather than by capacitance value (this is true of tanta-

lums, OS-CONs, polymers, and other electrolytics).

When using low-capacity filter capacitors, such as

ceramic capacitors, size is usually determined by the

capacity needed to prevent VSAG and VSOAR from

causing problems during load transients. Generally,

once enough capacitance is added to meet the over-

shoot requirement, undershoot at the rising load edge

is no longer a problem (see the VSAG and VSOAR equa-

tions in the Transient Response section). However, low-

capacity filter capacitors typically have high-ESR zeros

that may affect the overall stability (see the Output-

Capacitor Stability Considerations section).

METHOD

a) Output current-sense resistor

CURRENT-SENSE

ACCURACY

High

INDUCTOR-SATURATION

PROTECTION

Allowed

(highest accuracy)

CURRENT-SENSE POWER LOSS

(EFFICIENCY)

RSENSE x IOUT2

b) Low-side current-sense resistor

c) Low-side MOSFET on-resistance

d) Equivalent inductor DC resistance

High

Low

Not allowed

(LSAT = GND)

Not allowed

(LSAT = GND)

Allowed

ââ1-

VOUT

VIN

â â

RSENSE

IOUT2

No additional loss

______________________________________________________________________________________ 37

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