ADP2450ASTZ-5-R7 datasheet(27/40 Pages) AD

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Data Sheet

ADP2450

Rev. B | Page 27 of 40

EXTERNAL MOSFET FOR BOOST SHUNT

CONTROLLER

An N-channel external MOSFET is needed to control the CT

current in the boost shunt controller.

When the external MOSFET is turned off, the current from the CT

charges the output capacitor to VOUT1 through the boost shunt

diode. The voltage added on the drain and source nodes of the

MOSFET is equal to VOUT1 plus the diode forward voltage.

When the external MOSFET is turned on, it bypasses the CT

current to ground. It is recommended to choose a MOSFET

with a breakdown voltage (VDSS) at least twice that of the output

voltage of boost shunt controller (VOUT1). It is also recommended

that the continuous drain current (ID) be larger than the CT

secondary root mean square (rms) current when the analog trip

occurs.

The MOFSET driver integrated in the ADP2450 has an 8 V output

high voltage (VDRV_H). Ensure that the gate to source voltage (VGS) of

the selected MOSFET is greater than 8 V, and that the gate

threshold voltage (VGS_TH) is lower than 8 V. Table 12 lists several

recommended MOSFETs for the boost shunt controller.

Table 12. Recommended External MOSFETs

Vendor

Part Number

VDSS

(V)

(A)

Infineon

IRFR3505PBF

IRFR3518TRPBF

BSC340N08NS3GATMA1

DIODES

DMN6068LK3-13

8.5

DMN6013LFG-7

10.3

DMT8012LFG-13

ON Semiconductor

FDMC86340

NTTFS5820NLTAG

FDS5670

FDS3572

8.9

BOOST SHUNT DIODE SELECTION

The ADP2450 integrates a boost shunt controller that requires

an external Schottky rectifier to conduct the CT current to the

output capacitor of the boost shunt circuit when the external

boost shunt MOSFET is turned off. Ensure that the Schottky

diode peak current rating is larger than the maximum CT

secondary current. The peak reverse voltage of the Schottky

diode must be greater than the output voltage of boost shunt

controller. To achieve the best efficiency, select a Schottky diode

with a low forward voltage (VF).

Table 13. Recommended Schottky Diodes

Vendor

Part Number

VRRM1 (V)

IO2 (A)

DIODES

B360A

B350A

B260A

ON Semiconductor

MBRS360BT3G

MBRS260T3G

NRVBS260T3G

Rohm

RB055LAM-60TR

RB068LAM-60TR

Bourns

CD214A-B360LF

1 VRRM is the peak repetitive reverse voltage of the diodes.

2 IO is the forward current of the diodes.

INPUT CAPACITOR OF BUCK REGULATOR

The input capacitor reduces the input voltage ripple of the buck

regulator caused by the switching current on VIN. Place the

input capacitor as close as possible to the VIN pin. A 10 μF

ceramic capacitor is recommended. The loop that is composed

of this input capacitor, the high-side N-MOSFET, and the low-

side N-MOSFET must be kept as small as possible.

The voltage rating of the input capacitor must be greater than

the maximum input voltage. Ensure that the rms current rating

of the input capacitor is larger than the value calculated from

the following equation:



CIN RMS

OUT



 

where:

ICIN_RMS is the rms current of the input capacitor of buck regulator.

IOUT2 is the output current of the buck regulator.

D is the duty cycle of the buck regulator (D = VOUT2/VIN).

INDUCTOR SELECTION

The inductor value of the buck regulator is determined by the

operating frequency, input voltage, output voltage, and inductor

ripple current. Using a small inductor leads to a faster transient

response but degrades efficiency due to a larger inductor ripple

current, whereas using a large inductor value leads to smaller

ripple current and improved efficiency but results in a slower

transient response.

As a guideline, the inductor ripple current, ΔIL, is typically set

to one-third of the maximum load current. The inductor value

is calculated using the following equation:





OUT

LSW









where:

VIN is the input voltage of the buck regulator.

VOUT2 is the output voltage of the buck regulator.

IL is the inductor current ripple.

fSW is the switching frequency of buck regulator.

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