Have a problem with the output voltage of the charge pump circuit, LTC3261. The output is lower than expected

Question

The circuittis as the one in the data sheet with the following components

CGA5L2X8R1E105K

Cap Ceramic 1uF 25V X8R 10% Pad SMD 1206 +150&deg;C Automotive T/R

LTC3261MPMSE#PBF

High Voltage, Low Quiescent Current Inverting Charge Pump, 4.5 to 32 V Vin, 12-pin SOP (MSE-12-EDP), -50 to 150 degC, Pb-Free

C3225X8R1E106K250AC

Multilayer Ceramic Capacitors 10uF &plusmn;10% 25V X8R SMD 1210

The output voltages are as follows:

LTC

Temp

V IN

I IN

V O

T Plane

18 o C

12V

40mA

-11.06

19.6

70

12V

40mA

-10.9

69.8

80

12V

40mA

-10.88

80.4

90

12V

40mA

-10.84

90.5

100

12V

40mA

-10.83

100.9

110

12V

40mA

-10.8

111

120

12V

40mA

-10.76

120.8

130

12V

40mA

-10.72

130.7

140

12V

40mA

-10.68

140.6

150

12V

40mA

-10.63

150.4

Can you please tell me why the output voltage is not the inverse of the input voltage.

MartyM · Accepted Answer

Hello, 
 The open loop resistance of the LTC3261 does get higher with temperature. The output voltage is about -(VIN - IOUT*ROL) where ROL is the open loop resistance. Below is the open loop resistance over temperature. 
 
 With the temperature near 150&deg;C the open loop resistance is close to 50&Omega;. This will create a voltage loss of about 2V. I would expect the output voltage to be about -10V with a 12V input and 40mA. At 18&deg;C the open loop resistance should be over 30&Omega;. I would expect the voltage loss to be greater than 1.2V and the output voltage would be less than -11V in amplitude. These numbers are slightly better than what I would expect.)

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Have a problem with the output voltage of the charge pump circuit, LTC3261. The output is lower than expected