LTC3260
9
3260fa
operaTion
(Refer to the Block Diagram)
Figure 1. Oscillator Frequency vs R
T
The charge transfer frequency can be adjusted between
50kHz and 500kHz using an external resistor on the RT
pin. At slower frequencies the effective open-loop output
resistance (R
OL
) of the charge pump is larger and it is able
to provide smaller average output current. Figure 1 can
be used to determine a suitable value of RT to achieve a
required oscillator frequency. If the RT pin is grounded,
the part operates at a constant frequency of 500kHz.
R
T
(kΩ)
200
OSCILLATOR FREQUENCY (kHz)
400
600
100
300
500
1 100 1000 10000
3260 F01
0
10
Charge Pump Burst Mode Operation
The LTC3260 provides low power Burst Mode operation
when a logic high is applied to the MODE pin. In Burst
Mode operation, the charge pump charges the V
OUT
pin to
–0.94 • V
IN
(typical). The part then shuts down the internal
oscillator to reduce switching losses and goes into a low
current state. This state is referred to as the sleep state in
which the IC consumes only about 100µA with both LDOs
enabled. When the output voltage droops enough to over-
come the burst comparator hysteresis, the part wakes up
and commences charge pump cycles until output voltage
exceeds –0.94 • V
IN
(typical). This mode provides lower
operating current at the cost of higher output ripple and
is ideal for light load operation.
The frequency of charging cycles is set by the external
resistor on the RT pin. The charge pump has a lower
R
OL
at higher frequencies. For Burst Mode operation it is
recommended that the RT pin be tied to GND. This mini-
mizes the charge pump R
OL
, quickly charges the output
up to the burst threshold and optimizes the duration of
the low current sleep state.
Charge Pump Soft-Start
The LTC
3260 has built in soft-start circuitry to prevent
excessive
current flow during start-up. The soft-start is
achieved by internal circuitry that slowly ramps the amount
of current available at the output storage capacitor. The
soft-start circuitry is reset in the event of a commanded
shutdown or thermal shutdown.
Charge Pump Short-Circuit/Thermal Protection
The LTC3260 has built-in short-circuit current limit as
well as overtemperature protection. During a short-circuit
condition, the part automatically limits its output current
to approximately 160mA. If the junction temperature
exceeds approximately 175°C the thermal shutdown
circuitry disables current delivery to the output. Once
the junction temperature drops back to approximately
165°C current delivery to the output is resumed. When
thermal protection is active the junction temperature is
beyond the specified operating range. Thermal protection
is intended for momentary overload conditions outside
normal operation. Continuous operation above the speci-
fied maximum operating junction temperature may impair
device reliability.
Positive Low Dropout Linear Regulator (LDO
+
)
The positive low dropout regulator (LDO
+
) supports a load
of up to 50mA. The LDO
+
takes power from the V
IN
pin
and drives the LDO
+
output pin to a voltage programmed
by the resistor divider connected between the LDO
+
, ADJ
+
and GND pins. For stability, the LDO
+
output must be by-
passed to ground with a low ESR ceramic capacitor that
maintains a capacitance of at least 2µF across operating
temperature and voltage.
The LDO
+
is enabled or disabled via the EN
+
logic input pin.
When the LDO
+
is enabled, a soft-start circuit ramps its
regulation point from zero to the final value over a period
of 75µs, reducing the inrush current on V
IN
.
