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LTM4650A-1

4650a1f

For more information www.linear.com/LTM4650A-1

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4650a1 F11

125

ID = 100µA

Figure 11. Diode Voltage VD vs Temperature T(K)

for Different Bias Currents

APPLICATIONS INFORMATION

First the SW pin can be monitored with a wide bandwidth

scope with a high frequency scope probe. The ring fre-

quency can be measured for its value. The impedance Z

can be calculated:

Z(L) = 2πfL,

where f is the resonant frequency of the ring, and L is the

total parasitic inductance in the switch path. If a resistor

is selected that is equal to Z, then the ringing should be

dampened. The snubber capacitor value is chosen so that

its impedance is equal to the resistor at the ring frequency.

Calculated by: Z(C) = 1/(2πfC). These values are a good

place to start with. Modification to these components

should be made to attenuate the ringing with the least

amount of power loss.

Temperature Monitoring

A diode connected PNP transistor is used for the TEMP

monitor function by monitoring its voltage over tempera-

ture. The temperature dependence of this diode voltage

can be understood in the equation:

VD =nVT ln













where VT is the thermal voltage (kT/q), and n, the ideality

factor, is 1 for the diode connected PNP transistor being

used in the LTM4650A-1. IS is expressed by the typical

empirical equation:

IS =I0 exp

–VG0

















where I0 is a process and geometry dependent current, (I0

is typically around 20k orders of magnitude larger than IS

at room temperature) and VG0 is the band gap voltage of

1.2V extrapolated to absolute zero or –273°C.

If we take the IS equation and substitute into the VD equa-

tion, then we get:

VD = VG0 –

















, VT =

The expression shows that the diode voltage decreases

(linearly if I0 were constant) with increasing temperature

and constant diode current. Figure 11 shows a plot of VD

vs Temperature over the operating temperature range of

the LTM4650A-1.

If we take this equation and differentiate it with respect to

temperature T, then:

dVD

= –

VG0 – VD

This dVD/dT term is the temperature coefficient equal to

about –2mV/K or –2mV/°C. The equation is simplified for

the first order derivation.

Solving for T, T = –(VG0 – VD)/(dVD/dT) provides the

temperature.

1st Example: Figure 11 for 27°C, or 300K the diode

voltage is 0.598V, thus, 300K = –(1200mV – 598mV)/

–2.0 mV/K)

2nd Example: Figure 11 for 75°C, or 350K the diode

voltage is 0.50V, thus, 350K = –(1200mV – 500mV)/

–2.0mV/K)

Converting the Kelvin scale to Celsius is simply taking the

Kelvin temp and subtracting 273 from it.

A typical forward voltage is given in the electrical charac-

teristics section of the data sheet, and Figure 11 is the plot

of this forward voltage. Measure this forward voltage at

27°C to establish a reference point. Then using the above

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