LM3151 datasheet(17/27 Pages) TI1

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Pdh = 0.396 + 0.278 = 0.674W

Pcond = Iout

2 x R

DS(ON) x D

8.5

Vcc - Vth

6.8

Vth

x Vin x Iout x Qgd x fs x

Psw =

Pdh = Pcond + Psw

Pcond = 12

x 0.01 x 0.275 = 0.396W

8.5

6 ± 2.5

6.8

2.5

x 12 x 12 x 1.5 nC x 500 kHz x

Psw =

= 0.278W

LM3151, LM3152, LM3153

www.ti.com

SNVS562G – SEPTEMBER 2008 – REVISED MARCH 2011

Minimum output capacitance is:

COmin = 70 / (fS

2 x L)

COmin = 70 / (500 kHz

2 x 1.65 µH) = 169 µF

The maximum ESR allowed to prevent over-voltage protection during normal operation is:

ESRmax = (80 mV x L) / ET

ESRmax = (80 mV x 1.65 µH) / 5.7 V µs

ESRmax = 23 mΩ

The minimum ESR must meet both of the following criteria:

ESRmin ≥ (15 mV x L) / ET

ESRmin ≥ [ET / (VIN - VOUT)] / CO

ESRmin ≥ (15 mV x 1.65 µH) / 5.7 V µs = 4.3 mΩ

ESRmin ≥ [5.7 V µs / (12 - 3.3)] / 169 µF = 3.9 mΩ

Based on the above criteria two 150 µF polymer aluminum capacitors with a ESR = 12 m

Ω each for a effective

ESR in parallel of 6 m

Ω was chosen from Panasonic. The part number is EEF-UE0J151P.

5. MOSFET Selection

The LM3151/2/3 are designed to drive N-channel MOSFETs. For a maximum input voltage of 24V we should

choose N-channel MOSFETs with a maximum drain-source voltage, VDS, greater than 1.2 x 24V = 28.8V. FETs

with maximum VDS of 30V will be the first option. The combined total gate charge Qgtotal of the high-side and low-

side FET should satisfy the following:

Qgtotal ≤ IVCCL / fs

(4)

Qgtotal ≤ 65 mA / 500 kHz

(5)

Qgtotal ≤ 130 n

where

•

IVCCL is the minimum current limit of VCC over the temperature range, specified in the electrical characteristics

table

The MOSFET gate charge Qg is gathered from reading the VGS vs Qg curve of the MOSFET datasheet at the

VGS = 5V for the high-side, M1, MOSFET and VGS = 6V for the low-side, M2, MOSFET.

The Renesas MOSFET RJK0305DPB has a gate charge of 10 nC at VGS = 5V, and 12 nC at VGS = 6V. This

combined gate charge for a high-side, M1, and low-side, M2, MOSFET 12 nC + 10 nC = 22 nC is less than 130

nC calculated Qgtotal.

The calculated MOSFET power dissipation must be less than the max allowed power dissipation, Pdmax, as

specified in the MOSFET datasheet. An approximate calculation of the FET power dissipated Pd, of the high-side

and low-side FET is given by:

High-Side MOSFET

The max power dissipation of the RJK0305DPB is rated as 45W for a junction temperature that is 125°C higher

than the case temperature and a thermal resistance from the FET junction to case,

θJC, of 2.78°C/W.

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