MAX17005 查看數據表（PDF） - Maxim Integrated

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MAX17005

1.2MHz Low-Cost, High-Performance Chargers

Maxim Integrated 

1.2MHz Low-Cost,

High-Performance Chargers

For optimum size and inductor current ripple, choose

LIRMAX = 0.4, which sets the ripple current to 40% the

charge current and results in a good balance between

inductor size and efficiency. Higher inductor values

decrease the ripple current. Smaller inductor values

save cost but require higher saturation current capabili-

ties and degrade efficiency.

Inductor L1 must have a saturation current rating of at

least the maximum charge current plus 1/2 the ripple

current (ΔIL):

ISAT = ICHG + (1/2) ΔIL

The ripple current is determined by:

ΔIL = k × VIN2

4L

Input Capacitor Selection

The input capacitor must meet the ripple current

requirement (IRMS) imposed by the switching currents.

Nontantalum chemistries (ceramic, aluminum, or

OS-CON) are preferred due to their resilience to power-

up and surge currents:

( ) ⎛

IRMS = ICHG × ⎜

VBATT ×

VDCIN − VBATT

⎞

⎟

⎝⎜

VDCIN

⎠⎟

The input capacitors should be sized so that the tem-

perature rise due to ripple current in continuous con-

duction does not exceed approximately 10°C. The

maximum ripple current occurs at 50% duty factor or

VDCIN = 2 x VBATT, which equates to 0.5 x ICHG. If the

application of interest does not achieve the maximum

value, size the input capacitors according to the worst-

case conditions.

Output Capacitor Selection

The output capacitor absorbs the inductor ripple cur-

rent and must tolerate the surge current delivered from

the battery when it is initially plugged into the charger.

As such, both capacitance and ESR are important

parameters in specifying the output capacitor as a filter

and to ensure the stability of the DC-to-DC converter

(see the Compensation section.) Beyond the stability

requirements, it is often sufficient to make sure that the

output capacitor’s ESR is much lower than the battery’s

ESR. Either tantalum or ceramic capacitors can be

used on the output. Ceramic devices are preferable

because of their good voltage ratings and resilience to

surge currents. Choose the output capacitor based on:

COUT

=

fSW

IRIPPLE

× 8 × ΔVBATT

× kCAP−BIAS

Choose kCAP-BIAS is a derating factor of 2 for typical 25V-

rated ceramic capacitors.

For fSW = 800kHz, IRIPPLE = 1A, and to get ΔVBATT =

70mV, choose COUT as 4.7μF.

If the internal resistance of battery is close to the ESR of

the output capacitor, the voltage ripple is shared with

the battery and is less than calculated.

Applications Information

Setting Input Current Limit

The input current limit should be set based on the cur-

rent capability of the AC adapter and the tolerance of

the input current limit. The upper limit of the input cur-

rent threshold should never exceed the adapter’s mini-

mum available output current. For example, if the

adapter’s output current rating is 5A ±10%, the input

current limit should be selected so that its upper limit is

less than 5A × 0.9 = 4.5A. Since the input current-limit

accuracy of the MAX17005/MAX17006/MAX17015 is

±3%, the typical value of the input current limit should

be set at 4.5A/1.03 ≈ 4.36A. The lower limit for input

current must also be considered. For chargers at the

low end of the spec, the input current limit for this

example could be 4.36A × 0.95 or approximately 4.14A.

Layout and Bypassing

Bypass DCIN with a 0.1μF ceramic to ground (Figure 1).

N1 and N2 protect the MAX17005/MAX17006/

MAX17015 when the DC power source input is reversed.

Bypass VAA, CSSP, and LDO as shown in Figure 1.

Good PCB layout is required to achieve specified noise

immunity, efficiency, and stable performance. The PCB

layout designer must be given explicit instructions—

preferably, a sketch showing the placement of the

power switching components and high current routing.

Refer to the PCB layout in the MAX17005/MAX17006/

MAX17015 evaluation kit for examples. A ground plane

is essential for optimum performance. In most applica-

tions, the circuit is located on a multilayer board, and

full use of the four or more copper layers is recom-

mended. Use the top layer for high-current connec-

tions, the bottom layer for quiet connections, and the

inner layers for an uninterrupted ground plane.

Use the following step-by-step guide:

1) Place the high-power connections first, with their

grounds adjacent:

a) Minimize the current-sense resistor trace lengths,

and ensure accurate current sensing with Kelvin

connections.

b) Minimize ground trace lengths in the high-current

paths.

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