LTC3200 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3200

Low Noise, Regulated Charge Pump DC/DC Converters

Linear Technology 

LTC3200/LTC3200-5

U

OPERATIO

Operation (Refer to Simplified Block Diagrams)

The LTC3200/LTC3200-5 use a switched capacitor charge

pump to boost VIN to a regulated output voltage. Regula-

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

switches. The flying capacitor is charged from VIN on the

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

it is stacked in series with VIN and connected to VOUT. This

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

VIN supply. Furthermore, VOUT is disconnected from VIN.

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

damage until the short-circuit on VOUT is removed.

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 5µA when the

LTC3200 is in shutdown. However, this current will be

eliminated when the output voltage (VOUT) is at 0V. To

ensure that VOUT is at 0V in shutdown on the adjustable

LTC3200 a bleed resistor may be needed from VOUT to GND.

Typically 10k to 100k is acceptable.

Soft-Start

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

to prevent excessive current flow at VIN during start-up.

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:

ISTARTUP

=

2C OUT

VOUT

1ms

For example, with a 2.2µF output capacitor the start-up

input current of an LTC3200-5 will be approximately

22mA. If the output capacitor is 10µF 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:

R1 = VOUT – 1

R2 1.268V

Typical values for total voltage divider resistance can

range from several kΩs up to 1MΩ.

8

VOUT

7

FB

4

PGND

SGND 5

R1

( ) VOUT

1.268V 1 + R1

R2

COUT

R2

32005 F01

Figure 1. Programming the Adjustable LTC3200

6

Share Link: