LTC3252_15 Datasheet, PDF (6/12 Pages) Linear Technology – Dual, Low Noise, Inductorless Step-Down DC/DC Converter

LTC3252
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OPERATIO (Refer to Simplified Block Diagram)
The LTC3252 has two switched capacitor charge pumps to
step down V_IN to two regulated output voltages. The two
charge pumps operate 180° out of phase to reduce input
ripple. Regulation is achieved by sensing each output
voltage through an external resistor divider and modulat-
ing the charge pump output current based on the error
signal. A 2-phase nonoverlapping clock activates the two
charge pumps running them out of phase from each other.
On the first phase of the clock current is transferred from
V_IN, through the external flying capacitor 1, to OUT1 via the
switches of charge pump 1. Not only is current being
delivered to OUT1 on the first phase, but the flying capaci-
tor is also being charged up. On the second phase of the
clock, flying capacitor 1 is connected from OUT1 to
ground, transferring the charge stored during the first
phase of the clock to OUT1 via the switches of charge
pump 1. Charge pump 2 operates in the same manner to
supply current to OUT2, but with the phases of the clock
reversed relative to charge pump 1. Using this method of
switching, only half of the output current for each output
is delivered from V_IN, thus achieving a 50% increase in
efficiency over a conventional LDO. A spread spectrum
oscillator, which utilizes random switching frequencies
between 1MHz and 1.6MHz, sets the rate of charging and
discharging of the flying capacitors. This architecture
achieves extremely low output noise. Input noise is signifi-
cantly reduced compared to conventional charge pumps.
The outputs also have a low current burst mode to improve
efficiency even at light loads.
In shutdown mode all circuitry is turned off and the
LTC3252 draws only leakage current from the V_IN supply.
Furthermore, OUT1 and OUT2 are disconnected from V_IN.
The EN1 and EN2 pins are CMOS inputs with threshold
voltages of approximately 0.8V to allow regulator control
with low voltage logic levels. The LTC3252 is in shutdown
when a logic low is applied to both enable pins. Since the
mode pins are high impedance CMOS inputs, they should
never be allowed to float. Always drive the enable pins with
valid logic levels.
Short-Circuit/Thermal Protection
The LTC3252 has built-in short-circuit current limiting as
well as over temperature protection. During short-circuit
conditions, internal circuitry automatically limits each
output to approximately 500mA of current. If fault condi-
tions (such as shorted outputs) cause excessive self
heating on chip such that the junction temperature ex-
ceeds approximately 160°C, the thermal shutdown cir-
cuitry will disable the charge pumps. The IC resumes
operation once the junction temperature drops back to
approximately 155°C. The LTC3252 will cycle in and out of
thermal shutdown without latchup or damage until the
overstress condition is removed. Long term overstress
(I_OUT1 or I_OUT2 > 400mA, and/or T_J > 125°C) should be
avoided as it can degrade the performance or shorten the
life of the part.
Soft-Start
To prevent excessive current flow at V_IN during start-up,
the LTC3252 has built-in soft-start circuitry on each out-
put. When an output is enabled, the soft-start circuitry
increases the amount of current available from the output
linearly over a period of approximately 500µs. The soft-
start circuitry is disabled shortly after the output achieves
regulation.
Spread Spectrum Operation
Switching regulators can be particularly troublesome where
electromagnetic interference (EMI) is concerned. Switch-
ing regulators operate on a cycle-by-cycle basis to transfer
power to an output. In most cases, the frequency of
operation is either fixed or is a constant based on the
output load. This method of conversion creates large
components of noise at the frequency of operation (funda-
mental) and multiples of the operating frequency (har-
monics).
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