LTC1986 Datasheet, PDF (5/8 Pages) Linear Technology – 3V/5V SIM Power Supply in SOT-23

LTC1986
APPLICATIONS INFORMATION
Operation
The LTC1986 is a charge pump DC/DC converter designed
specifically to supply V_CC power for either 3V or 5V SIM
cards. The part can operate in one of three modes: a low
power shutdown mode which disconnects V_IN from V_OUT
and actively pulls down the V_OUT supply, 3V mode which
provides a direct connection between V_IN and V_OUT 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 V_IN on phase
one of the clock. On phase two of the clock, C1 is stacked
in series with V_IN and connected to V_OUT 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 V_IN:
Shutdown Mode ⇒ V_MODE < V_IN/2 – 0.1V
5V Mode
⇒ V_IN/2 + 0.1V < V_MODE < V_IN – 1V
3V Mode
⇒ V_IN – 0.25V < V_MODE
For example, if V_IN = 3V, then the following table applies:
V_MODE
0V to 1.4V
1.6V to 2V
2.75V to 3V
OPERATING MODE
Shutdown Mode
5V Mode
3V Mode (V_OUT = V_IN)
The three modes can be selected using two totem pole
digital outputs and two external resistors as shown in
Figure 1.
Shutdown mode disconnects V_IN from V_OUT and enables
an internal pull-down device which forces V_OUT 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 V_OUT to V_IN through a 2.5Ω
internal switch. The LTC1986 has the additional capability
of boosting V_OUT to a regulated 2.9V in 3V mode when the
input voltage drops below 2.85V. This ensures that the 3V
SIM V_CC specification is not violated if V_IN drops below
2.7V during transients or low battery conditions.
V_IN
3/5
SHDN
6
V_IN
LTC1986
1M
3 _MODE
750k
SHDN 3/5
MODE
0 0 SHUTDOWN (I_CC < 1µA)
0 1 SHUTDOWN (I_CC = 10µA)
10
V_OUT = 5V
11
V_OUT = V_IN
1986 F01
Figure 1
Capacitor Selection
For best performance, it is recommended that low ESR
capacitors be used for both C_IN and C_OUT to reduce noise
and ripple. The C_IN and C_OUT 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 (< 0.5Ω), C_IN may not be needed.
For best performance, C_OUT should be 2.2µF or greater.
Ceramic capacitors are also recommended for the flying
capacitor C1 with values between 0.047µF and 0.22µF. If
a small value output capacitor (3.3µF 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: C_IN = 0.1µF,
C1 = 0.047µF and C_OUT = 2.2µF (ceramic).
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