LTC3261
OPERATION
(Refer to the Block Diagram)
The LTC3261 is a high voltage inverting charge pump. It
supports a wide input power supply range from 4.5V to 32V.
Shutdown Mode
In shutdown mode, all circuitry except the internal bias is
turned off. The LTC3261 is in shutdown when a logic low is
applied to the enable input (EN). The LTC3261 only draws
2μA (typical) from the V
IN
supply in shutdown.
Constant Frequency Operation
The LTC3261 provides low noise constant frequency opera-
tion when a logic low is applied to the MODE pin. The charge
pump and oscillator circuit are enabled using the EN pin. At
the beginning of a clock cycle, switches S1 and S2 are closed.
The external flying capacitor across the C
+
and C
–
pins is
charged to the V
IN
supply. In the second phase of the clock
cycle, switches S1 and S2 are opened, while switches S3
and S4 are closed. In this configuration the C
+
side of the
flying capacitor is grounded and charge is delivered through
the C
–
pin to V
OUT
. In steady state the V
OUT
pin regulates at
–V
IN
less any voltage drop due to the load current on V
OUT
.
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.
Burst Mode Operation
The LTC3261 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 60μA. When the output
voltage droops enough to overcome 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 minimizes the charge pump
R
OL
, quickly charges the output up to the burst threshold
and optimizes the duration of the low current sleep state.
600
OSCILLATOR FREQUENCY (kHz)
500
400
300
200
100
0
1
10
100
RT (kΩ)
1000
10000
3261 F01
Figure 1. Oscillator Frequency vs RT
Soft-Start
The LTC3261 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.
Short-Circuit/Thermal Protection
The LTC3261 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 ex-
ceeds 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 mo-
mentary overload conditions outside normal operation.
Continuous operation above the specified maximum op-
erating junction temperature may impair device reliability.
3261f
7
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
