LTC3261
OPERATION (Refer to the Block Diagram)
The LTC3261 is a high voltage inverting charge pump. It
supportsawideinputpowersupplyrangefrom4.5Vto32V.
Thefrequencyofchargingcyclesissetbytheexternalresistor
on the RT pin. The charge pump has a lower R at higher
OL
frequencies.ForBurstModeoperationitisrecommendedthat
Shutdown Mode
the RT pin be tied to GND. This minimizes the charge pump
R , quickly charges the output up to the burst threshold
OL
In shutdown mode, all circuitry except the internal bias is
turnedoff. TheLTC3261isinshutdownwhenalogiclowis
applied to the enable input (EN). The LTC3261 only draws
and optimizes the duration of the low current sleep state.
600
2μA (typical) from the V supply in shutdown.
IN
500
400
300
200
100
0
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
thebeginningofaclockcycle,switchesS1andS2areclosed.
+
–
The external flying capacitor across the C and C pins is
charged to the V supply. In the second phase of the clock
IN
1
10
100
1000
10000
cycle, switches S1 and S2 are opened, while switches S3
RT (kΩ)
+
and S4 are closed. In this configuration the C side of the
3261 F01
flying capacitor is grounded and charge is delivered through
Figure 1. Oscillator Frequency vs RT
–
the C pin to V . In steady state the V
pin regulates at
OUT
OUT
–V less any voltage drop due to the load current on V
.
IN
OUT
Soft-Start
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
The LTC3261 has built in soft-start circuitry to prevent
excessive current flow during start-up. The soft-start is
achievedbyinternalcircuitrythatslowlyrampstheamount
of current available at the output storage capacitor. The
soft-start circuitry is reset in the event of a commanded
shutdown or thermal shutdown.
resistance (R ) of the charge pump is larger and it is able
OL
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.
Short-Circuit/Thermal Protection
Burst Mode Operation
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-
ceedsapproximately175°Cthethermalshutdowncircuitry
disables current delivery to the output. Once the junction
temperature drops back to approximately 165°C current
deliverytotheoutputisresumed.Whenthermalprotection
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.
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
pin to
OUT
–0.94 • V (typical). The part then shuts down the internal
IN
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,thepartwakesupandcommenceschargepump
cycles until output voltage exceeds –0.94 • V (typical).
IN
This mode provides lower operating current at the cost of
higher output ripple and is ideal for light load operation.
3261f
7
Linear Systems [ Linear Systems ]
