LTC1682CMS8-5 データシートの表示（PDF） - Linear Technology

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LTC1682CMS8-5

Doubler Charge Pumps with Low Noise Linear Regulator

Linear Technology 

LTC1682/LTC1682-3.3/LTC1682-5

APPLICATIONS INFORMATION

The following formula can be used to find the maximum

output voltage that may be programmed for a given

minimum input voltage and output current load:

VOUT(MAX) = (2)(VIN(MIN)) – (IOUT)(RCPO) – VDROPOUT

with the condition that (IOUT)(RCPO) < 0.55VIN.

Example:

VIN(MIN) = 3V

IOUT = 10mA

RCPO(MAX) = 20Ω

Max unloaded CPO voltage = 6V

Loaded CPO voltage = 6V – (10mA)(20Ω) = 5.8V

VDROPOUT(MAX) = 0.08V

VOUT(MAX) = (6V) – (0.2V) – (0.08V) = 5.72V

VOUT < 5.5V and (IOUT)(RCPO) < 0.55VIN, 0.2V < 1.65V.

For minimum noise applications, the LDO must be kept out

of dropout to prevent CPO noise from coupling into VOUT.

External CPO Loading

The CPO output can drive an external load (an LDO, for

example). The current required by this additional load will

reduce the available current from VOUT. If the external load

requires 5mA, then the maximum available current at VOUT

will be reduced by 5mA.

Short-Circuit and Thermal Protection

VOUT can be shorted to ground indefinitely. Internal cir-

cuitry will limit the output current. If the junction tempera-

ture exceeds 150°C, the part will shut down. Excessive

power dissipation due to heavy loads will also cause the

part to shut down when the junction temperature exceeds

150°C. The part will become enabled when the junction

temperature drops below 140°C. If the fault condition

remains in place, the part will cycle between the shutdown

and enabled states.

Capacitor Selection

For best performance it is recommended that low ESR

capacitors be used for C2, C3 and C4 in Figure 1 to

reduce noise and ripple. C2 must be ≥ 2µF and C3 must

be equal to or greater than C2. C4 is dependent on the

source impedance. The charge pump demands large

instantaneous currents which may induce ripple onto

a common voltage rail. C4 should be ≥ 2µF and a spike

reducing resistor of 2.2Ω may be required between

VIN and the supply.

A low ESR ceramic capacitor is recommended for the

flying capacitor C1 with a value of 0.22µF. At low load or

high VIN a smaller capacitor could be used to reduce ripple

on CPO which would reflect as lower ripple on VOUT.

If a minimum enable time is required, the CPO output filter

capacitor should be at least 2× the VOUT filter capacitor.

When the LDO is first enabled, the CPO capacitor will

dump a large amount of charge into the VOUT capacitor. If

the drop in the CPO voltage falls below 1.45(VIN), the LDO

will be disabled and the CPO voltage will have to charge up

to 1.75(VIN) to enable the LDO. The resulting cycling

extends the enable time.

A 1nF filter capacitor for the LTC1682-3.3/LTC1682-5

should be connected between the FILT pin and ground for

optimum noise performance.

Output Ripple

The output noise and ripple on CPO includes a spike

component from the charge pump switches and a droop

component which is dependent on the load current and the

value of C3. The charge pump has been carefully designed

to minimize the spike component; however, low ESR

capacitors are essential to reduce the remaining spike

energy effect on the CPO voltage. C3 should be increased

for high load currents to minimize the droop component.

Ripple components on CPO are greatly reduced at VOUT by

the LDO; however, C2 should also be a low ESR capacitor

to improve filtering of the CPO noise.

Shutdown

When SHDN pin is pulled low (< 0.4V), the part will be in

shutdown, the supply current will be < 5µA and VOUT will

be connected to ground through a 100Ω switch. In addi-

tion, CPO will be high impedance and disconnected from

VIN.

If shutdown is not required, connect SHDN to VIN which

will continuously enable the part.

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