uP9303

14

uP9303-DS-F0101, Aug. 2017

www.upi-semi.com

Power MOSFET Selection

External component selection is primarily determined by

the maximum load current and begins with the selection

of power MOSFET switches. The uP9303 requires two

external N-channel power MOSFETs for upper (controlled)

and lower (synchronous) switches. Important parameters

for the power MOSFETs are the breakdown voltage V

on-resistance R

maximum current I

thermal management requirements.

The gate drive voltage is powered by VCC pin that receives

10.8V~13.2V supply voltage. When operating with a 12V

power supply for VCC (or down to a minimum supply

voltage of 8V), a wide variety of NMOSFETs can be used.

Logic-level threshold MOSFET should be used if the input

voltage is expected to drop below 8V. Since the lower

MOSFET is used as the current sensing element,

particular attention must be paid to its on-resistance. Look

for R

Special cautions should be exercised on the lower switch

exhibiting very low threshold voltage V

through protection present aboard the uP9303 may be

circumvented by these MOSFETs if they have large

parasitic impedances and/or capacitances that would

inhibit the gate of the MOSFET from being discharged

below its threshold level before the complementary

MOSFET is turned on. Also avoid MOSFETs with

excessive switching times; the circuitry is expecting

transitions to occur in under 50 nsec or so.

In high-current applications, the MOSFET power

dissipation, package selection and heatsink are the

dominant design factors. The power dissipation includes

two loss components; conduction loss and switching loss.

The conduction losses are the largest component of power

dissipation for both the upper and the lower MOSFETs.

These losses are distributed between the two MOSFETs

according to duty cycle. Since the uP9303 is operating in

continuous conduction mode, the duty cycles for the

MOSFETs are:

IN

OUT

UP

V

V

D

=

;

IN

OUT

IN

LO

V

V

V

D

−

=

The resulting power dissipation in the MOSFETs at

maximum output current are:

OSC

SW

IN

OUT

UP

)

ON

(

DS

2

OUT

UP

f

T

V

I

5

.

0

D

R

I

P

×

×

×

×

+

×

×

=

LO

)

ON

(

DS

2

OUT

LO

D

R

I

P

×

×

=

Application Information

includes an additional term for switching losses, which

are largest at high input voltages. The bottom MOSFET

losses are greatest when the bottom duty cycle is near

88%, during a short-circuit or at high input voltage. These

equations assume linear voltage current transitions and

do not adequately model power loss due the reverse-

recovery of the lower MOSFET’s body diode. Ensure that

both MOSFETs are within their maximum junction

temperature at high ambient temperature by calculating

the temperature rise according to package thermal-

resistance specifications. A separate heatsink may be

necessary depending upon MOSFET power, package

type, ambient temperature and air flow.

The gate-charge losses are dissipated by the uP9303 and

don’t heat the MOSFETs. However, large gate charge

MOSFET switching losses. The gate-charge losses are

calculated as:

OSC

RSS

IN

LO

_

ISS

UP

_

ISS

CC

CC

G

f

)

C

V

)

C

C

(

V

(

V

P

×

×

+

+

×

×

=

of the upper MOSFET. Make sure that the gate-charge

loss will not cause over temperature at uP9303, especially

with large gate capacitance and high supply voltage.

Output Inductor Selection

Output inductor selection usually is based the

considerations of inductance, rated current, size

requirement, and DC resistance (DC)

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple

current:

)

V

V

1

(

V

L

f

1

I

IN

OUT

OUT

OUT

OSC

L

−

×

×

×

=

∆

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a tradeoff between

component size, efficiency and operating frequency. A

reasonable starting point is to choose a ripple current that

There is another tradeoff between output ripple current/

voltage and response time to a transient load. Increasing

the value of inductance reduces the output ripple current

and voltage. However, the large inductance values reduce

the converter’s response time to a load transient.

Maximum current ratings of the inductor are generally