ADP3182 datasheet(16/20 Pages) AD

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ADP3182

Rev. 0 | Page 16 of 20

The closest standard 1% value for RPH(X) is 140 kΩ. Next, use

Equation 7 to solve for CCS.

100

600

≥

Choose the closest standard value that is greater than the result

given by Equation 7. This example uses a CCS value of 5.6 nF.

OUTPUT VOLTAGE

ADP3182 has an internal FBRTN voltage reference VREF of

800 mV. The output voltage can be set up using a voltage

divider made up of resistors RB1 and RB2:

REF

OUT

(8)

Rearranging Equation 8 to solve for RB2 using the ADP3182

with an internal FB voltage of 800 mV and assuming a 1%, 1 kΩ

resistor for RB1 yields

Ω

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

REF

OUT

The closest standard 1% resistor value for RB2 is 1.24 kΩ.

POWER MOSFETS

For this example, one high-side, N-channel power MOSFET

and two low-side, N-channel power MOSFETs per phase have

been selected. The main selection parameters for the power

MOSFETs are VGS(TH), QG, CISS, CRSS, and RDS(ON). The minimum

gate drive voltage (the supply voltage to the ADP3418) dictates

whether standard threshold or logic-level threshold MOSFETs

must be used. With VGATE ~10 V, logic-level threshold MOSFETs

(VGS(TH) < 2.5 V) are recommended.

The maximum output current (IO) determines the RDS(ON)

requirement for the low-side (synchronous) MOSFETs. With

the ADP3182, currents are balanced between phases, thus the

current in each low-side MOSFET is the output current divided

by the total number of MOSFETs (nSF). With conduction losses

being dominant, the following expression shows the total power

being dissipated in each synchronous MOSFET in terms of the

ripple current per phase (IR) and the average total output current

(IO):

()

( )

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

(9)

Knowing the maximum output current and the maximum

allowed power dissipation, one can determine the required

RDS(ON) for the MOSFET. For D-PAK MOSFETs up to an

ambient temperature of 50°C, a safe limit for PSF is 1 W to 1.5 W

at 120°C junction temperature. Therefore, for this example,

RDS(SF) (per MOSFET) < 7.5 mΩ. This RDS(SF) is also at a junction

temperature of about 120°C, so one must account for this when

making this selection. This example uses a lower-side MOSFET

with 4.8 mΩ at 120°C.

Another important factor for the synchronous MOSFET is the

input capacitance and feedback capacitance. The ratio of

feedback to input must be small (less than 10% is recom-

mended) to prevent accidentally turning on the synchronous

MOSFETs when the switch node goes high.

Also, the time to switch the synchronous MOSFETs off should

not exceed the nonoverlap dead time of the MOSFET driver

(40 ns typical for the ADP3418). The output impedance of the

driver is approximately 2 Ω, and the typical MOSFET input gate

resistances are about 1 Ω to 2 Ω; therefore, one should adhere to

a total gate capacitance of less than 6000 pF. Because there are

two MOSFETs in parallel, the input capacitance for each

synchronous MOSFET should be limited to 3000 pF.

The high-side (main) MOSFET must handle two main power

dissipation components: conduction and switching losses. The

switching loss is related to the amount of time for the main

MOSFET to turn on and off, and to the current and voltage that

are being switched. Basing the switching speed on the rise and

fall time of the gate driver impedance and MOSFET input

capacitance, the following expression provides an approximate

value for the switching loss per main MOSFET:

()

ISS

= 2

(10)

where:

nMF is the total number of main MOSFETs.

RG is the total gate resistance (2 Ω for the ADP3418 and about

1 Ω for typical high speed switching MOSFETs, making RG = 3 Ω).

CISS is the input capacitance of the main MOSFET.

Note that adding more main MOSFETs (nMF) does not help the

switching loss per MOSFET because the additional gate

capacitance slows switching. The most efficient way to reduce

switching loss is to use lower gate capacitance devices.

The conduction loss of the main MOSFET is given by the

following equation:

()

(

)

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ ×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(11)

where:

RDS(MF) is the on resistance of the MOSFET.

Typically, for main MOSFETs, the highest speed (low CISS)

device is preferred, but faster devices usually have higher on

resistance. Select a device that meets the total power dissipation

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