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LTC1986

1986fa

APPLICATIONS INFORMATION

Operation

The LTC1986 is a charge pump DC/DC converter designed

specifically to supply VCC power for either 3V or 5V SIM

cards. The part can operate in one of three modes: a low

power shutdown mode which disconnects VIN from VOUT

and actively pulls down the VOUT supply, 3V mode which

provides a direct connection between VIN and VOUT for

powering 3V SIM cards, and 5V mode which provides a

regulated 5V output for powering 5V SIM cards. In 5V

mode, regulation is achieved by sensing the output volt-

age through a resistor divider and enabling the charge

pump as needed to maintain regulation.

When the charge pump is enabled, a 2-phase

nonoverlapping clock controls the internal charge pump

switches. Flying capacitor C1 is charged to VIN on phase

one of the clock. On phase two of the clock, C1 is stacked

in series with VIN and connected to VOUT through an

internal switch. This sequence of charging and discharg-

ing the flying capacitor occurs at a free-running frequency

of 900kHz (typ) and continues until the divided output

voltage reaches the upper trip point of the comparator.

Once the output is in regulation, the charge pump is

disabled. This method of bursting the charge pump on and

off enables the LTC1986 to achieve high efficiency at

extremely low output loads.

Mode Selection

The operating mode is determined by the voltage applied

to the MODE pin. The MODE pin has three voltage thresh-

olds—all of which are referenced to VIN:

Shutdown Mode

⇒ VMODE < VIN/2 – 0.1V

5V Mode

⇒ VIN/2 + 0.1V < VMODE < VIN – 1V

3V Mode

⇒ VIN – 0.25V < VMODE

For example, if VIN = 3V, then the following table applies:

VMODE

OPERATING MODE

0V to 1.4V

Shutdown Mode

1.6V to 2V

5V Mode

2.75V to 3V

3V Mode (VOUT = VIN)

The three modes can be selected using two totem pole

digital outputs and two external resistors as shown in

Figure 1.

Shutdown mode disconnects VIN from VOUT and enables

an internal pull-down device which forces VOUT to GND.

To achieve the lowest possible quiescent current in shut-

down mode, the voltage on the MODE pin should be pulled

below 0.4V. 3V mode connects VOUT to VIN through a 2.5Ω

internal switch. The LTC1986 has the additional capability

of boosting VOUT to a regulated 2.9V in 3V mode when the

input voltage drops below 2.85V. This ensures that the 3V

SIM VCC specification is not violated if VIN drops below

2.7V during transients or low battery conditions.

Figure 1

Capacitor Selection

For best performance, it is recommended that low ESR

capacitors be used for both CIN and COUT to reduce noise

and ripple. The CIN and COUT capacitors should be either

ceramic or tantalum. Ceramic capacitors will provide the

smallest size for a given capacitance. If the input source

impedance is very low (< 0.5

Ω), CIN may not be needed.

For best performance, COUT should be 2.2µF or greater.

Ceramic capacitors are also recommended for the flying

capacitor C1 with values between 0.047

µF and 0.22µF. If

a small value output capacitor (3.3

µF or less) is used, a

small flying capacitor should also be used to prevent high

output ripple in 5V mode. The recommended capacitor

values to minimize board space and cost are: CIN = 0.1µF,

C1 = 0.047

µF and COUT = 2.2µF (ceramic).

MODE

VIN

LTC1986

750k

3/5

SHDN

1986 F01

SHDN

3/5

MODE

SHUTDOWN (ICC < 1µA)

SHUTDOWN (ICC = 10µA)

VOUT = 5V

VOUT = VIN

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