ADP3210 datasheet(29/38 Pages) ONSEMI

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ADP3210

Rev. 0.3 | Page 29 of 38

For example, using an IRF7821 device as the main MOSFET

(four in total; that is, nMF = 4), with about CISS = 1010 pF (max)

and RDS(MF) = 18 mΩ (max at TJ = 120°C) and an IR7832 device

as the synchronous MOSFET (four in total; that is, nSF = 4),

RDS(SF) = 6.7 mΩ (max at TJ = 120°C). Solving for the power

dissipation per MOSFET at IO = 32 A and IR = 10.7 A yields

420 mW for each synchronous MOSFET and 410 mW for each

main MOSFET.

One last consideration is the power dissipation in the driver for

each phase. This is best described in terms of the QG for the

MOSFETs and is given by the following equation:

()

GSF

GMF

DRV

⎥⎦

⎤

⎢⎣

⎡

(20)

where:

QGMF is the total gate charge for each main MOSFET.

QGSF is the total gate charge for each synchronous MOSFET.

Also shown is the standby dissipation (ICC × VCC) of the driver.

For the ADP3419, the maximum dissipation should be less than

300 mW, considering its thermal impedance is 220°C/W and

the maximum temperature increase is 50°C. For this example,

with ICC = 2 mA, QGMF = 14 nC and QGSF = 51 nC, there is 120

mW dissipation in each driver, which is below the 300 mW

dissipation limit. See the ADP3419 data sheet for more details.

RAMP RESISTOR SELECTION

The ramp resistor (RR) is used for setting the size of the internal

PWM ramp. The value of this resistor is chosen to provide the

best combination of thermal balance, stability, and transient

response. Use this equation to determine a starting value

462

360

(21)

where:

AR is the internal ramp amplifier gain.

AD is the current balancing amplifier gain.

RDS is the total low-side MOSFET ON-resistance,

CR is the internal ramp capacitor value.

Another consideration in the selection of RR is the size of the

internal ramp voltage (see Equation 22). For stability and noise

immunity, keep this ramp size larger than 0.5 V. Taking this into

consideration, the value of RR is selected as 280 kΩ.

The internal ramp voltage magnitude can be calculated using:

kHz

280

462

150

)

061

(

)

(

−

VID

(22)

The size of the internal ramp can be made larger or smaller. If it

is made larger, then stability and transient response improves,

but thermal balance degrades. Likewise, if the ramp is made

smaller, then thermal balance improves at the sacrifice of

transient response and stability. The factor of three in the

denominator of Equation 21 sets a minimum ramp size that

gives an optimal balance for good stability, transient response,

and thermal balance.

COMP Pin Ramp

There is a ramp signal on the COMP pin due to the droop

voltage and output voltage ramps. This ramp amplitude adds to

the internal ramp to produce the following overall ramp signal

at the PWM input:

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

(23)

For this example, the overall ramp signal is found to be 1.5 V.

SETTING THE SWITCHING FREQUENCY FOR RPM

MODE OPERATION OF PHASE 1

During the RPM mode operation of Phase 1, the ADP3210 runs

in pseudo constant frequency, given that the load current is

high enough for continuous current mode. While in

discontinuous current mode, the switching frequency is

reduced with the load current in a linear manner. When

considering power conversion efficiency in light load, lower

switching frequency is usually preferred for RPM mode.

However, the VCORE ripple specification in the IMVP-6 sets the

limitation for lowest switching frequency. Therefore, depending

on the inductor and output capacitors, the switching frequency

in RPM mode can be equal, larger, or smaller than its

counterpart in PWM mode.

A resistor from RPM to GND sets the pseudo constant

frequency as following:

−

)

(

VID

RPM

(24)

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