LTC3250-1.5/LTC3250-1.2

6

3250fa

The LTC3250-1.5/LTC3250-1.2 use a switched capacitor

1.2V

±4% (respectively) output voltage. Regulation is

achieved by sensing the output voltage through an internal

resistor divider and modulating the charge pump output

current based on the error signal. A 2-phase nonoverlapping

clock activates the charge pump switches. On the first

tor is also being charged up. On the second phase of the

ground, delivering the charge stored during the first phase

achieving twice the efficiency over a conventional LDO.

The sequence of charging and dis-charging the flying

capacitor continues at a free running frequency of 1.5MHz

(typ). This constant frequency architecture provides a low

noise regulated output as well as lower input noise than

conventional switch-capacitor charge pump regulators.

The part also has a low current Burst Mode operation to

improve efficiency even at light loads.

In shutdown mode all circuitry is turned off and the

LTC3250-1.5/LTC3250-1.2 draw only leakage current from

voltage of approximately 0.8V. The LTC3250-1.5/LTC3250-

1.2 are in shutdown when a logic low is applied to the

SHDN pin. Since the SHDN pin is a high impedance CMOS

input it should never be allowed to float. To ensure that its

state is defined it must always be driven with a valid logic

level.

Short-Circuit/Thermal Protection

The LTC3250-1.5/LTC3250-1.2 have built-in short-circuit

current limiting as well as overtemperature protection.

During short-circuit conditions, the parts will automati-

cally limit the output current to approximately 500mA. At

higher temperatures, or if the input voltage is high enough

to cause excessive self heating on chip, thermal shutdown

circuitry will shut down the charge pump once the junction

temperature exceeds approximately 160

°C.Itwillreenable

the charge pump once the junction temperature drops

back to approximately 150

°C.TheLTC3250-1.5/LTC3250-

1.2 will cycle in and out of thermal shutdown without latch-

should be avoided as it can degrade the performance of the

part.

Soft-Start

the LTC3250-1.5/LTC3250-1.2 have a built-in soft-start

circuitry. Soft-start is achieved by increasing the amount

of current available to the output charge storage capacitor

linearly over a period of approximately 500

µs. Soft-start is

enabled whenever the device is brought out of shutdown,

and is disabled shortly after regulation is achieved.

Low Current “Burst Mode” Operation

To improve efficiency at low output currents, Burst Mode

operation was included in the design of the LTC3250-1.5/

LTC3250-1.2. An output current sense is used to detect

when the required output current drops below an inter-

nally set threshold (30mA typ.). When this occurs, the part

shuts down the internal oscillator and goes into a low

current operating state. The LTC3250-1.5/LTC3250-1.2

will remain in the low current operating state until the

output has dropped enough to require another burst of

current. Unlike traditional charge pumps whose burst

current is dependant on many factors (i.e. supply voltage,

switch resistance, capacitor selection, etc.), the LTC3250-

1.5/LTC3250-1.2’s burst current is set by the burst thresh-

in Burst Mode will be fixed and is typically 12mV for a

4.7

µF output capacitor.

Power Efficiency

The power efficiency (

η) of the LTC3250-1.5/LTC3250-

1.2 are approximately double that of a conventional linear

regulator. This occurs because the input current for a 2 to

1 step-down charge pump is approximately half the output

OPERATIO

(Refer to Simplified Block Diagram)