LTC3355 - Charge Current and Current Limit

Question

Accepted Answer

Issue: 
 The circuit diagram on page 1 of the datasheet shows that the supercap charger is fed from the Vout voltage, but the block diagram on page 9 does not show a connection between the Vout and the supercap charger. This leads to confusion about how to calculate the maximum charge current for a given load since the datasheet suggests up to 1A of load current and up to 1A of charge current. However, these two things cannot occur at the same time. 
 
 Page 1 
 
 Page 9 
 Explanation: 
 The supercap LDO charger is fed from circuitry connected to Vout. Vout is supplied by the buck regulator. The buck regulator has a SW current limit (page 3, minimum limit of 1.3A), the supercap LDO charge current can be limited by the charge current resistor, and the chip has an input current limit (set by the resistor connected to Vin and Vins). 
 
 The average current through the inductor is the same as the load current drawn from the chip. However, this load current includes the contribution of the LDO charger and the external load since both are loading the buck. These two currents impact the input current and the max SW current. The max SW current is the same as the maximum inductor current. The maximum inductor current is: 
 
 Where: 
 
 For example, for the demo board with Vout = 5V, Vin = 12V, and assuming 0.7V forward voltage for the diode, that would be: 
 
 Thus, with a max SW current of 1.3A, the maximum total load current would be determined by: 
 
 Or: 
 
 Plugging in the calculated values, if the load tries to draw 1A from Vout: 
 
 Thus, the supercap charger current limit would need to be set to a maximum 56mA to keep the chip from hitting the SW current limit. If the SW current limit is hit due to setting the charge current limit too high, then the Vout will collapse. 
 
 Additionally, a larger inductor would allow for the supercap charge current to increase because increasing the inductor size decreases the ripple current, allowing a larger load to be drawn before hitting the 1.3A SW current limit. For example, a 13uH inductor would result in a ripple of about 0.255A, so the inductor current ripple in the previous equation would be about 256mA, allowing for 172mA of supercap charge current with 1A of load current before hitting the SW current limit. However, if the ripple is decreased too much, the chip will not be able to function properly. This is because the chip, which is based on a current-mode architecture, will not be able to sense the inductor ripple current which the control circuitry uses to regulate the output voltage. 
 
 In the end, the maximum charge current of the supercap while providing a load at the output depends on the load current that is drawn and the inductor ripple current. Use the formula: 
 
 Rearrange to: 
 
 In order to to determine the maximum possible supercap charge current for a specific application. If a higher supercap charge current is desired, set the supercap charge current limit to be higher, charge the supercap before turning on the load so all of the buck current can feed the supercap, and then disable the supercap charging after it is fully charged so that the supercap charge current does not violate the maximum SW current.

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LTC3355: FAQ