MAX1902 datasheet(21/33 Pages) MAXIM

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500kHz Multi-Output, Low-Noise Power-Supply

Controllers for Notebook Computers

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connected but no battery is installed. VIN(MAX) must not

exceed 30V.

Minimum Input (Battery) Voltage, VIN(MIN).This should

be taken at full load under the lowest battery condi-

tions. If VIN(MIN) is less than 4.2V, use an external cir-

cuit to externally hold VL above the VL undervoltage

lockout threshold. If the minimum input-output differ-

ence is less than 1.5V, the filter capacitance required to

maintain good AC load regulation increases (see the

Low-Voltage Operation section).

Inductor Value

The exact inductor value isn’t critical and can be freely

adjusted to make trade-offs between size, cost, and

efficiency. Lower inductor values minimize size and

cost, but reduce efficiency due to higher peak-current

levels. The smallest inductor is achieved by lowering

the inductance until the circuit operates at the border

between continuous and discontinuous mode. Further

reducing the inductor value below this crossover point

results in discontinuous-conduction operation even at

full load. This helps lower output-filter capacitance

requirements, but efficiency suffers due to high I2R

losses. On the other hand, higher inductor values mean

greater efficiency, but resistive losses due to extra wire

turns will eventually exceed the benefit gained from

lower peak-current levels. Also, high inductor values

can affect load-transient response (see the VSAG equa-

tion in the Low-Voltage Operation section). The equa-

tions that follow are for continuous-conduction

operation, since the MAX1901/MAX1902/MAX1904 are

intended mainly for high-efficiency, battery-powered

applications. Discontinuous conduction doesn’t affect

normal idle-mode operation.

Three key inductor parameters must be specified:

inductance value (L), peak current (IPEAK), and DC

resistance (RDC). The following equation includes a

constant (LIR) which is the ratio of inductor peak-to-

peak AC current to DC load current. A higher LIR value

allows smaller inductance, but results in higher losses

and higher ripple. A good compromise between size

and losses is found at a 30% ripple-current to load-cur-

rent ratio (LIR = 0.3), which corresponds to a peak-

inductor current 1.15 times higher than the DC load

current.

where:

f = switching frequency, normally 333kHz or

500kHz

IOUT = maximum DC load current

LIR = ratio of AC to DC inductor current, typi-

cally 0.3; should be >0.15

The nominal peak-inductor current at full load is 1.15 ✕

IOUT if the above equation is used; otherwise, the peak

current can be calculated by:

The inductor’s DC resistance should be low enough that

RDC ✕ IPEAK < 100mV, as it is a key parameter for effi-

ciency performance. If a standard off-the-shelf inductor is

not available, choose a core with an LI2 rating greater

than L ✕ IPEAK2 and wind it with the largest-diameter wire

that fits the winding area. Ferrite core material is strongly

preferred. Shielded-core geometries help keep noise,

EMI, and switching-waveform jitter low.

Current-Sense Resistor Value

The current-sense resistor value is calculated accord-

ing to the worst-case low current-limit threshold voltage

(from the Electrical Characteristics) and the peak

inductor current:

Use IPEAK from the second equation in the Inductor

Value section.

Use the calculated value of RSENSE to size the MOS-

FET switches and specify inductor saturation-current

ratings according to the worst-case high current-limit

threshold voltage:

Low-inductance resistors, such as surface-mount

metal-film, are recommended.

Input-Capacitor Value

The input filter capacitor is usually selected according

to input ripple current requirements and voltage rating,

rather than capacitor value. Ceramic capacitors or

Sanyo OS-CON capacitors are typically used to handle

the power-up surge-currents, especially when connect-

ing to robust AC adapters or low-impedance batteries.

RMS input ripple current (IRMS) is determined by the

input voltage and load current, with the worst case

occurring at VIN = 2 ✕ VOUT:

PEAK MAX

SENSE

() =

120

SENSE

PEAK

LOAD

OUT

IN MAX

OUT

IN MAX

=+ ()

×× ×

(

()

LIR

OUT

IN MAX

OUT

IN MAX

OUT

()

××

()

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