Texas Instruments

Hold-Up Time & Hold-Up Capacitor Calculator

Purpose:

The tool calculates the required hold-up capacitance to enable full operation of a system during a period of input power absence. The calculation takes the system's input voltage and output power, the capacitor's tolerances, the efficiency and UVLO of the system's DC/DC converter into account. Beside of the capacitance calculation the tool can also be used to solve for the other system parameters listed above in the case that the user specifies a specific capacitance value.

Use of the tool:

First the desired result should be selected out of the respective list field. According to that selection, the fields will be marked as entry fields (yellow) or result field (grey). The units of the input fields can be changed by use of the list boxes assigned to the input fields. Default values which are used by the tool for some of the entry fields, will be shown in bold blue. The default values can be over-written by the user.

Capacitor tolerances need to be entered as positive values, the tool calculates the worst-case scenario treating positive numbers as negative tolerances.

As soon as all entry fields have been filled with valid numbers, the tool calculates the result.

In the case the user has selected to calculate the “Hold-up Capacitance”, the tool solves first for a minimum required capacitance value and proposes also a value out of the E12 series for use. The capacitance value proposed for use is the next E12 value larger than the minimum required value. In the case that the next E12 value lower than the minimum required value results in an achieved hold-up time which is at least 95% of the required hold-up time, this lower E12 value will be proposed. In each case the user can increase / decrease the used E12 capacitance value step-by-step simply by clicking on the “+” or “-“ buttons below the used hold-up capacitance field. In addition, the value can be completely over-written by the user with a non-E12 value.

Mathematics behind:

Calculations are based on the following basic equation:

Where

Cis the hold-up capacitance
ηis the efficiency of the DC/DC converter
VINis the input voltage which equals to the capacitor's voltage under normal operation
VUVLO-OFFis the undervoltage lockout level of the DC/DC converter. Under the condition of VIN absence, the DC/DC converter discharges the hold-up capacitor until the capacitor voltage reaches the VUVLO-OFF
POUTis the power provided by the DC/DC converter to the load
THold-Upis the time the DC/DC converter is able to provide the specified POUT in the case of VIN absence
TOLare the initial, temperature and aging tolerances of the capacitor (please use positive numbers only for specifying the tolerances)

References:

Juergen Schneider; How to use TI's Set of Power Rulers; Texas Instruments Application Report (SLVA278) – September 2007