MAQ3203 View Datasheet(PDF) - Micrel

Part Name

Description

View to exact match

MAQ3203

High-Brightness LED Driver Controller with High-Side Current Sense

Micrel 

Micrel, Inc.

MAQ3203

The switching loss occurs during the MOSFET turn-on

and turn-off transition and can be found by:

PLOSS(TRAN)

=

VIN × ILED × FSW

IDRV

× (Qgs2

+ Qgd )

IDRV

=

VDRV

RGATE

where:

RGATE is total MOSFET resistance, Qgs2 and Qgd can be

found in a MOSFET manufacturer datasheet.

The total power loss is:

PLOSS(TOT) = PLOSS(CON) + PLOSS(TRAN)

The MOSFET junction temperature is given by:

TJ = PLOSS(TOT) × RθJA + TA

The TJ must not exceed maximum junction temperature

under any conditions.

Snubber

A RC voltage snubber is used to damp out high

frequency ringing on the switch node caused by parasitic

inductance and capacitance. The capacitor is used to

slow down the switch node rise and fall time and the

resistor damps the ringing. Excessive ringing can cause

the MAQ3203 to operate erratically by prematurely

tripping its current limit comparator circuitry.

The snubber is connected across the Schottky diode as

shown in the evaluation board schematic. Capacitor CS

(C4) is used to block the DC voltage across the resistor,

minimizing the power dissipation in the resistor. This

capacitor value should be between two to five times the

parasitic capacitance of the MOSFET COSS and the

Schottky diode junction capacitance Cj. A capacitor that

is too small will have high impedance and prevent the

resistor from damping the ringing. A capacitor that is too

large causes unnecessary power dissipation in the

resistor, which lowers efficiency.

The snubber components should be placed as close as

possible to the Schottky diode. Placing the snubber too

far from the diode or using an etch that is too long or too

thin adds inductance to the snubber and diminishes its

effectiveness.

Proper snubber design requires the parasitic inductance and

capacitance be known. A method of determining these

values and calculating the damping resistor value is outlined

below:

1. Measure the ringing frequency at the switch node which

is determined by parasitic LP and CP. Define this

frequency as f1.

2. Add a capacitor CS (normally at least 3 times as big as

the COSS of the diode) across the diode and measure the

new ringing frequency. Define this new (lower) frequency

as f2. LP and CP can now be solved using the values of

f1, f2 and CS.

3. Add a resistor RS in series with CS to generate critical

damping. If the snubber resistance is equal to the

characteristic impedance of the resonant circuit

(1/sqrt(LPCP)), the resonant circuit will be critically

damped and have no ringing.

Step 1: First measure the ringing frequency on the switch

node voltage when the high-side MOSFET turns on. This

ringing is characterized by the equation:

f1 =

2π

1

LP ×CP

where:

CP and LP are the parasitic capacitance and inductance.

Step 2: Add a capacitor, CS, in parallel with the Schottky

diode. The capacitor value should be approximately 3 times

the COSS of D1. Measure the frequency of the switch node

ringing, f2.

f2 =

2π

1

LP ×(CS + CP )

Define f’ as:

f' = f1

f2

Combining the equations for f1, f2 and f’ to derive CP, the

parasitic capacitance:

⎜⎛

⎜⎝

CP

=

CS

2 × (f,)2

⎟⎞

− 1⎟⎠

March 2011

13

M9999-032411-A

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