LM7372IMAXSLASHNOPB datasheet(19/28 Pages) TI1

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LM7372

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SNOS926F – MAY 1999 – REVISED SEPTEMBER 2014

Figure 26 shows that there could be 1 mA or more excess supply current per amplifier with close to full output

swing (24 VPP) when frequency is just above 1MHz (or at higher frequencies when the output swing is less). This

boost in supply current enables the output to “keep up” with high frequency/large signal output swing, but in turn,

increases the total package power dissipation and therefore raises the device junction temperature. As a

consequence, it is necessary to pay special attention to the package heatsink design for these demanding

applications, especially for ones that run at higher supply voltages. For that reason, Figure 26 has the safe

operating limits for the 8-Pin SO PowerPAD package -- for example, “30V supply (2 amplifiers)” horizontal line --

superimposed on top of it (with TJ limit of 140°C when operated at 85°C ambient), so that the designer can

readily decide whether or not there is need for additional heat sinking.

For example, if the LM7372 is operating similarly to the Figure 24 schematic with a single power supply of 10 V,

it is safe to have up to 10 VPP output swing at up to 40 MHz with no additional heat sinking. This is determined

by inspecting Figure 24 where the "10 V supply (2 amplifiers)" safe operating limit intercepts the 10 VPP swing

graph at around 40 MHz Use the "10 V supply (1 amplifier) safe operating limit in cases where the second

amplifier in the LM7372 package does not experience high frequency/high output swing conditions.

At any given “IS increase” value (y axis), the product of frequency and output swing remains essentially constant

for all output swing plots. This holds true for the lower frequency range before the plots experience a slope

increase. Therefore, if the application example just discussed operates up to 60MHz instead, it is possible to

calculate the junction-temperature-limited maximum output swing of 6.7 VPP(= 40 MHz x 10VPP/60 MHz) instead.

Please note that Figure 26 precludes any additional amplifier power dissipation related to load (this topic is

discussed below in detail). This load current, if large enough, will reduce the operating frequency/output swing

further. It is important to note that the LM7372 can be destroyed if it is allowed to dissipate enough power that

compromises its maximum junction temperature limit of 150°C.

With the op amp tied to a load, the device power dissipation consists of the quiescent power due to the supply

current flow into the device, in addition to power dissipation due to the load current. The load portion of the

power itself could include an average value (due to a DC load current) and an AC component. DC load current

would flow if there is an output voltage offset, or the output AC average current is non-zero, or if the op amp

operates in a single supply application where the output is maintained somewhere in the range of linear

operation. Therefore:

PD(TOTAL) = PQ + PDC + PAC

(7)

PQ = |IS • VS| (Op Amp Quiescent Power Dissipation)

(8)

PDC = |IO • (VR - VO)| (DC Load Power)

(9)

For PAC, (AC Load Power) see Table 3

where:

•

IS = Supply Current

•

VS = Total Supply Voltage (V

+ - V−)

•

IO = Average Load Current

•

VO = Average Output Voltage

•

VR = Reference Voltage (V

+ for sourcing and V− for sinking current)

Table 3 shows the maximum AC component of the load power dissipated by the op amp for standard Sinusoidal,

Triangular, and Square Waveforms:

Table 3. Normalized Maximum AC Power Dissipated in the Output Stage for Standard Waveforms

PAC (W.Ω/V

SINUSOIDAL

TRIANGULAR

SQUARE

50.7 x 10−3

46.9 x 10−3

62.5 x 10−3

The table entries are normalized to VS

2/R

L. These entries are computed at the output swing point where the

amplifier dissipation is the highest for each waveform type. To figure out the AC load current component of

power dissipation, simply multiply the table entry corresponding to the output waveform by the factor VS

2/R

L. For

example, with ±5V supplies, a 100-

Ω load and triangular output waveform, power dissipation in the output stage

is calculated as: PAC = 46.9 x 10

−3 x 102/100 = 46.9mW which contributes another 2.2°C (= 46.9mW x 47°C/W)

rise to the LM7372 junction temperature in the 8-Pin SO PowerPAD package.

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