LTC3204-3.3/LTC3204-5/

LTC3204B-3.3/LTC3204B-5

7

3204fa

TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5

regulatedoutputvoltage.Regulationisachievedbysensing

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

chargepumpswitches.Theflyingcapacitorischargedfrom

flying capacitor continues at a free running frequency of

1.2MHz (typ).

Shutdown Mode

In shutdown mode, all circuitry is turned off and the

LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5

CMOSinputwithathresholdvoltageofapproximately0.7V.

TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5

are in shutdown when a logic low is applied to the SHDN

pin. Since the SHDN pin is a very 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.

Since the output voltages of these devices can go above

the input voltage, special circuitry is required to control

theinternallogic.Detectionlogicwilldrawaninputcurrent

of 5µA when the devices are in shutdown. However, this

less than approximately 0.8V.

Burst Mode Operation

The LTC3204-3.3/LTC3204-5 provide automatic Burst

Mode operation to reduce supply current at light loads.

Burst Mode operation is initiated if the output load current

falls below an internally programmed threshold. Once

(Refer to the Block Diagram)

Burst Mode operation is initiated, the part shuts down

the internal oscillator to reduce the 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 40µA

from the input. When the output voltage droops enough

to overcome the burst comparator hysteresis, the part

wakes up and commences normal fixed frequency opera-

tion. The output capacitor recharges and causes the part

to reenter the sleep state if the output load still remains

less than the Burst Mode threshold. This Burst Mode

storage capacitor.

Soft-Start

TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5

havebuilt-insoft-startcircuitrytopreventexcessivecurrent

flowduringstart-up.Thesoft-startisachievedbycharging

an internal capacitor with a very weak current source. The

voltage on this capacitor, in turn, slowly ramps the amount

of current available to the output storage capacitor from

zero to a value of 300mA over a period of approximately

0.75ms. The soft-start circuit is reset in the event of a

commanded shutdown or thermal shutdown.

Short-Circuit/Thermal Protection

TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5

havebuilt-inshort-circuitcurrentlimitaswellasover-tem-

perature protection. During a short-circuit condition, they

willautomaticallylimittheiroutputcurrenttoapproximately

300mA. At higher temperatures, or if the input voltage is

high enough to cause excessive self-heating of the part,

the thermal shutdown circuitry will shutdown the charge

pumponcethejunctiontemperatureexceedsapproximately

160°C. It will enable the charge pump once the junction

temperature drops back to approximately 150°C. The

LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5will

cycle in and out of thermal shutdown indefinitely without

latchup or damage until the short-circuit condition on

OPERATIO