MAX1635A Просмотр технического описания (PDF) - Maxim Integrated

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MAX1635A

Multi-Output, Low-Noise Power-Supply Controllers for Notebook Computers

Maxim Integrated 

Multi-Output, Low-Noise Power-Supply

Controllers for Notebook Computers

rate output of more than 120mA is needed, an external

pass transistor can be added. Figure 6’s circuit delivers

more than 200mA. Total output current is constrained

by the V+ input voltage and the transformer primary

load (see Maximum 15V VDD Output Current vs.

Supply Voltage graphs in the Typical Operating

Characteristics).

__________________Design Procedure

The three predesigned 3V/5V standard application cir-

cuits (Figure 1 and Table 1) contain ready-to-use solu-

tions for common application needs. Also, two standard

flyback transformer circuits support the 12OUT linear

regulator in the Applications Information section. Use

the following design procedure to optimize these basic

schematics for different voltage or current require-

ments. Before beginning a design, firmly establish the

following:

• Maximum input (battery) voltage, VIN(MAX). This

value should include the worst-case conditions,

such as no-load operation when a battery charger

or AC adapter is 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

conditions. If VIN(MIN) is less than 4.2V, use an

external circuit to externally hold VL above the VL

undervoltage lockout threshold. If the minimum

input-output difference is less than 1.5V, the filter

capacitance required to maintain good AC load

regulation increases (see Low-Voltage Operation

section).

Inductor Value

The exact inductor value is not 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-cur-

rent levels. The smallest inductor is achieved by lower-

ing 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 val-

ues 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 equation in the Low-Voltage Operation sec-

tion). The equations that follow are for continuous-con-

duction operation, since the MAX1630A family is

intended mainly for high-efficiency, battery-powered

applications. Refer to Appendix A in Maxim’s Battery

Management and DC-DC Converter Circuit Collection

for crossover-point and discontinuous-mode equations.

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-

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

inductor current 1.15 times higher than the DC load

current:

L = VOUT(VIN(MAX) - VOUT)

VIN(MAX) x f x IOUT x LIR

where:

f = switching frequency, normally 200kHz or

300kHz

IOUT = maximum DC load current

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

cally 0.3; should be selected for >0.15

The nominal peak inductor current at full load is 1.15 x

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

current can be calculated by:

IPEAK

=

ILOAD

+

VOUT

2x

(VIN(MAX) - VOUT

f x L x VIN(MAX)

)

The inductor’s DC resistance should be low enough that

RDC x 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 x IPEAK2 and wind it with the largest diameter

wire that fits the winding area. For 300kHz applications,

ferrite core material is strongly preferred; for 200kHz

applications, Kool-Mµ® (aluminum alloy) or even pow-

dered iron is acceptable. If light-load efficiency is unim-

portant (in desktop PC applications, for example), then

low-permeability iron-powder cores, such as the

Micrometals type found in Pulse Engineering’s 2.1µH

PE-53680, may be acceptable even at 300kHz. For

high-current applications, shielded-core geometries,

such as toroidal or pot core, help keep noise, EMI, and

switching-waveform jitter low.

Kool-Mµ is a registered trademark of Magnetics Div., Spang & Co.

18 ______________________________________________________________________________________

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