LTC4155 datasheet(12/26 Pages) AD

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LTC4041

Rev A

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OPERATION

The LTC4041 is a complete supercapacitor backup system

manager for a 2.9V to 5.5V supply rail. The system has

three principal circuit components: a full-featured step-

down (buck) supercapacitor charger, a step-up (boost)

backup converter to deliver power to the system load

when external input power is lost, and a power-fail com-

parator to decide which one to activate. The LTC4041 has

several other auxiliary components: an input current limit

(IMON) amplifier, an optional input overvoltage protec-

tion (OVP) circuit, and a system power good (SYSGD)

comparator.

The LTC4041 has three modes of operation: normal,

backup and shutdown. If the input supply is above an

externally programmable PFI threshold voltage, the

LTC4041 is considered to be in normal mode. In this

normal mode power flows from input to output (VSYS)

while the step-down switching regulator charges a super-

capacitor or a stack of supercapacitors to a charge voltage

programmed by an external resistor divider connected at

the CAPFB pin. Refer to the Block Diagram.

The total system load is monitored by the IMON amplifier

via an external series resistor, RS, connected between the

VIN and CLN pins. This amplifier can reduce the charge

current from its programmed value (set by the PROG

pin external resistor RPROG) if the external load demand

increases beyond the level set by RS.

When the input supply falls below the PFI threshold,

backup mode disconnects the switches (MN1 and MN2)

to isolate the system (VSYS) from the input, and the boost

converter powers the system load from the supercapacitor

using the external inductor, L1.

THE SUPERCAPACITOR CHARGER

The LTC4041 includes a full-featured constant-current

(CC)/ constant-voltage (CV) supercapacitor charger with

programmable charge current and charge voltage, auto-

matic recharge, supercapacitor good indicator, superca-

pacitor overvoltage detection, and an internal balancer.

The charger is a high efficiency, constant frequency

(2.25MHz) synchronous buck converter used to charge

SCAP from VSYS via the SW pin. It is capable of directly

charging the supercapacitor to its charge voltage with an

externally programmable charge current up to 2.5A from

an input supply as high as 5.5V. A zero current com-

parator monitors the inductor current and shuts off the

NMOS synchronous rectifier once the current reduces to

approximately 250mA. This prevents the inductor current

from reversing and improves efficiency for low charg-

ing currents. The charger can be disabled by pulling the

CHGEN pin above 1.2V.

Constant-Current Mode Charging

In constant-current (CC) mode, the average current

delivered to the supercapacitor can reach 2000V/ RPROG.

Depending on the external load condition, the superca-

pacitor charger may or may not be able to charge at the

full programmed rate. The external load will always be

prioritized over the supercapacitor charge current. The

charger will charge at the full programmed rate only if

the sum of the external load and the charger input current

is less than or equal to the input current limit set by RS.

If the buck charger is operating at very low duty cycles

(i.e. if the supercapacitor voltage is very low), the actual

average charge current delivered to the supercapacitor

could vary by as much as 50% of the programmed value.

At low duty cycles, the measurement accuracy of the

inductor current sensing circuitry in the CC servo loop is

low. As a result, the average charge current could over-

shoot or undershoot. When the supercapacitor (or a stack

of supercapacitors) is charged from 0V, the low accuracy

of the inductor current sensing causes the buck to operate

in discontinuous mode. As the SCAP voltage increases

the buck will try to servo the average charge current to

the programmed value. When the SCAP voltage is about

1V, the buck exits discontinuous mode and the average

charge current will be at the programmed level. During

this discontinuous mode of operation, the VSYS voltage

ripple is still well-controlled despite the large inductor cur-

rent ripple because the buck is running at a low duty cycle.

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