LTM4646 datasheet(23/30 Pages) AD

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LTM4638

Rev. C

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lowering output current or power with increasing ambient

temperature. The decreased output current will decrease

the internal module loss as ambient temperature is in-

creased. The monitored junction temperature of 120°C

minus the ambient operating temperature specifies how

much module temperature rise can be allowed. As an

example, in Figure 16 the load current is derated to ~8A

at ~92°C with no air flow or heat sink and the power loss

for the 12V to 1.5V at 8A output is about 1.8W. The 8W

loss is calculated with the ~1.5W room temperature loss

from the 12V to 1.5V power loss curve at 8A, and the 1.2

multiplying factor at 120°C junction temperature. If the

APPLICATIONS INFORMATION

Table 7. Output Voltage Response vs Component Matrix (Refer to Figure 23)

COUT1 VENDORS PART NUMBER

DESCRIPTION

COUT2

VENDORS

PART NUMBER DESCRIPTION

Murata

GRM186R60J226ME

22µF, 6.3V, X5R,

0603

PANASONIC EEF-GX0E471L 470µF, 2.5V, 3mΩ

TDK

C1608X5R0J226M080AC 22µF, 6.3V, X5R,

0603

Murata

GRM31CR60J107ME

100µF, 6.3V,

X5R, 1206

Taiyo Yuden

JMK316BJ107ML

100µF, 6.3V,

X5R, 1206

TDK

C3216X5R0J107M160AB 100µF, 6.3V,

X5R, 1206

All Ceramic Output Capacitors

VOUTn

(V)

VINn

(V)

RFB

(kΩ)

fSW

(kHz)

COUT1

(CERAMIC CAP)

COUT2

(BULK CAP)

COMPa

(pF)

RTH

COMPa

(kΩ)

CTH

COMPa

(pF)

LOAD

STEP

(A)

PK-PK

DEVIATION

(mV)

RECOVERY

TIME

(µs)

90.9

600

22µF + 3x100µF

None

Short to

COMPb

None

3.75

90.9

600

22µF + 3x100µF

None

Short to

COMPb

None

3.75

94.4

1.5

40.2

600

22µF + 3x100µF

None

Short to

COMPb

None

3.75

106.4

1.5

40.2

800

22µF + 3x100µF

None

Short to

COMPb

None

3.75

109.1

2.5

19.1

800

22µF + 3x100µF

None

Short to

COMPb

None

3.75

165.4

2.5

19.1

1000

22µF + 3x100µF

None

Short to

COMPb

None

3.75

171

3.3

13.3

800

22µF + 3x100µF

None

Short to

COMPb

None

3.75

218

3.3

13.3

1000

22µF + 3x100µF

None

Short to

COMPb

None

3.75

218

8.25

1500

22µF + 3x100µF

None

Short to

COMPb

None

3.75

328

92°C ambient temperature is subtracted from the 120°C

junction temperature, then the difference of 28°C divided

by 18W equals a 15.5°C/W θJA thermal resistance. Table 3

specifiesa15°C/Wvaluewhichisveryclose.Table4,Table5

and Table 6 provide equivalent thermal resistances for

1.5V 3.3V and 5V outputs with and without airflow and

heat sinking. The derived thermal resistances in Table 3,

Table 4, Table 5 and Table 6 for the various conditions can

be multiplied by the calculated power loss as a function

of ambient temperature to derive temperature rise above

ambient, thus maximum junction temperature. Room

temperature power loss can be derived from the efficiency

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