MAX669EUB/V 데이터 시트보기 (PDF) - Maxim Integrated

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MAX669EUB/V

1.8V to 28V Input, PWM Step-Up Controllers in µMAX

Maxim Integrated 

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

given output ripple. An inductance value larger than

LIDEAL may also be used, but output-filter capacitance

must be increased by the same proportion that L has to

LIDEAL. See the Capacitor Selection section for more

information on determining output filter values.

Due the MAX668/MAX669’s high switching frequencies,

inductors with a ferrite core or equivalent are recom-

mended. Powdered iron cores are not recommended

due to their high losses at frequencies over 50kHz.

Determining Peak Inductor Current

The peak inductor current required for a particular out-

put is:

ILPEAK = ILDC + (ILPP / 2)

where ILDC is the average DC input current and ILPP is

the inductor peak-to-peak ripple current. The ILDC and

ILPP terms are determined as follows:

ILDC

=

IOUT (VOUT + VD)

(VIN – VSW )

where VD is the forward voltage drop across the

Schottky rectifier diode (D1), and VSW is the drop

across the external FET, when on.

ILPP

=

(VIN – VSW ) (VOUT + VD – VIN)

L x fOSC (VOUT + VD)

where L is the inductor value. The saturation rating of

the selected inductor should meet or exceed the calcu-

lated value for ILPEAK, although most coil types can be

operated up to 20% over their saturation rating without

difficulty. In addition to the saturation criteria, the induc-

tor should have as low a series resistance as possible.

For continuous inductor current, the power loss in the

inductor resistance, PLR, is approximated by:

PLR ≅ (IOUT x VOUT / VIN)2 x RL

where RL is the inductor series resistance.

Once the peak inductor current is selected, the current-

sense resistor (RCS) is determined by:

RCS = 85mV / ILPEAK

For high peak inductor currents (>1A), Kelvin sensing

connections should be used to connect CS+ and

PGND to RCS. PGND and GND should be tied together

at the ground side of RCS.

Power MOSFET Selection

The MAX668/MAX669 drive a wide variety of N-channel

power MOSFETs (NFETs). Since LDO limits the EXT

output gate drive to no more than 5V, a logic-level

NFET is required. Best performance, especially at low

input voltages (below 5V), is achieved with low-thresh-

old NFETs that specify on-resistance with a gate-

source voltage (VGS) of 2.7V or less. When selecting an

NFET, key parameters can include:

1) Total gate charge (Qg)

2) Reverse transfer capacitance or charge (CRSS)

3) On-resistance (RDS(ON))

4) Maximum drain-to-source voltage (VDS(MAX))

5) Minimum threshold voltage (VTH(MIN))

At high switching rates, dynamic characteristics (para-

meters 1 and 2 above) that predict switching losses

may have more impact on efficiency than RDS(ON),

which predicts DC losses. Qg includes all capacitances

associated with charging the gate. In addition, this

parameter helps predict the current needed to drive the

gate at the selected operating frequency. The continu-

ous LDO current for the FET gate is:

IGATE = Qg x fOSC

For example, the MMFT3055L has a typical Qg of 7nC

(at VGS = 5V); therefore, the IGATE current at 500kHz is

3.5mA. Use the FET manufacturer’s typical value for Qg

in the above equation, since a maximum value (if sup-

plied) is usually too conservative to be of use in esti-

mating IGATE.

Diode Selection

The MAX668/MAX669’s high switching frequency

demands a high-speed rectifier. Schottky diodes are

recommended for most applications because of their

fast recovery time and low forward voltage. Ensure that

the diode’s average current rating is adequate using

the diode manufacturer’s data, or approximate it with

the following formula:

IDIODE

=

IOUT

+

ILPEAK -

3

IOUT

Also, the diode reverse breakdown voltage must

exceed VOUT. For high output voltages (50V or above),

Schottky diodes may not be practical because of this

voltage requirement. In these cases, use a high-speed

silicon rectifier with adequate reverse voltage.

Capacitor Selection

Output Filter Capacitor

The minimum output filter capacitance that ensures sta-

bility is:

C OUT(MIN)

=

(7.5V x L / L IDEAL)

(2πRCS x VIN(MIN) x fOSC )

where VIN(MIN) is the minimum expected input voltage.

Typically COUT(MIN), though sufficient for stability, will

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