LTC3200/LTC3200-5

6

OPERATIO

Operation (Refer to Simplified Block Diagrams)

The LTC3200/LTC3200-5 use a switched capacitor charge

tion is achieved by sensing the output voltage through an

internal resistor divider (LTC3200-5) and modulating the

charge pump output current based on the error signal. A

2-phase nonoverlapping clock activates the charge pump

first phase of the clock. On the second phase of the clock

sequence of charging and discharging the flying capacitor

continues at a free running frequency of 2MHz (typ).

In shutdown mode all circuitry is turned off and the

LTC3200/LTC3200-5 draw only leakage current from the

The SHDN pin is a CMOS input with a threshold voltage of

approximately 0.8V. The LTC3200/LTC3200-5 is in shut-

down 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 LTC3200/LTC3200-5 have built-in short-circuit current

limiting as well as overtemperature protection. During

short-circuit conditions, they will automatically limit their

output current to approximately 225mA. At higher tempera-

tures, or if the input voltage is high enough to cause exces-

sive self heating on chip, thermal shutdown circuitry will

shut down the charge pump once the junction temperature

exceeds approximately 160

°C. It will reenable the charge

pump once the junction temperature drops back to approxi-

mately 155

°C. The LTC3200/LTC3200-5 will cycle in and

out of thermal shutdown indefinitely without latch-up or

Shutdown Current

Since the output voltage can go above the input voltage,

special circuitry is required to control internal logic.

Detection logic will draw an input current of 5

µA when the

LTC3200 is in shutdown. However, this current will be

Typically 10k to 100k is acceptable.

Soft-Start

The LTC3200/LTC3200-5 have built-in soft-start circuitry

The soft-start time is preprogrammed to approximately

1ms, so the start-up current will be primarily dependent

upon the output capacitor. The start-up input current can

be calculated with the expression:

IC

V

ms

STARTUP

OUT

OUT

= 2

1

For example, with a 2.2

µF output capacitor the start-up

input current of an LTC3200-5 will be approximately

22mA. If the output capacitor is 10

µF then the start-up

input current will be about 100mA.

Programming the LTC3200 Output Voltage (FB Pin)

While the LTC3200-5 version has an internal resistive

divider to program the output voltage, the programmable

LTC3200 may be set to an arbitrary voltage via an external

resistive divider. Since it employs a voltage doubling

charge pump, it is not possible to achieve output voltages

greater than twice the available input voltage. Figure 1

shows the required voltage divider connection.

The voltage divider ratio is given by the expression:

R

R

V

V

OUT

1

2

1 268

1

=

.

–

Typical values for total voltage divider resistance can

range from several k

Ωs up to 1MΩ.

8

7

4

FB

PGND

R1

32005 F01

R2

5

1.268V 1 + R1

R2

()

SGND

Figure 1. Programming the Adjustable LTC3200