LT3650-4.1/LT3650-4.2

14

36504142fb

RNG/SS: Dynamic Charge Current Adjust

The LT3650 gives the user the capability to adjust maximum

charge current dynamically through the RNG/SS pin. The

voltage on the RNG/SS pin corresponds to ten times the

default maximum sense voltage is 100mV, so maximum

charge current can be expressed as:

no effect on the maximum charge current.

The LT3650 sources 50μA from the RNG/SS pin, such that

a current control voltage can be set by simply connecting

an appropriately valued resistor to ground, following the

relation:

R

V

A

RNG SS

RNG SS

/

/

=

50

μ

For example, to reduce the maximum charge current to 50%

of the original value, which corresponds to a maximum

sense voltage of 50mV, RNG/SS would be set to 0.5V.

R

V

A

k

RNG SS

/

.

==

05

50

10

μ

Ω

This feature could be used, for example, to switch in a

reduced charge current level. Active servos can also be

used to impose voltages on the RNG/SS pin, provided they

can only sink current. Active circuits that source current

cannot be used to drive the RNG/SS pin. Resistive pull-ups

can be used, but extreme care must be taken not to exceed

the 2.5V absolute maximum voltage on the pin.

RNG/SS: Soft-Start

Soft-start functionality is also supported by the RNG/SS

pin. 50μA is sourced from the RNG/SS pin, so connecting

ates a linear voltage ramp. The maximum charge current

follows this voltage. Thus, the charge current increases

from zero to the fully programmed value as the capacitor

the relation:

The RNG/SS pin is pulled to ground internally when charg-

ing is terminated so each new charging cycle begins with

a soft-start cycle. RNG/SS is also pulled to ground during

bad-battery and NTC fault conditions, so a graceful recovery

from these faults is possible.

APPLICATIONS INFORMATION

Figure 6. Using the RNG/SS Pin for Digital

Control of Maximum Charge Current

Figure 7. Driving the RNG/SS Pin with a

Current-Sink Active Servo Amplifier

Figure 8. Using the RNG/SS Pin for Soft-Start

365042 F06

RNG/SS

10k

LOGIC HIGH = HALF CURRENT

LT3650

365042 F08

RNG/SS

LT3650

365042 F07

RNG/SS

SERVO REFERENCE

LT3650

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–