AIC1848CG Просмотр технического описания (PDF) - Analog Intergrations

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AIC1848CG

Regulated 5V Charge Pump In SOT-23

Analog Intergrations 

AIC1848

VCFLY, that is, 2VIN.

VIN

Q1

Q2 VOUT

CIN

CFLY

COUT

Q3

Q4

Fig. 11 The circuit of charge pump

Short Circuit/Thermal Protection

AIC1848 obtains built-in short circuit current

limiting and over temperature protection. During

the short circuit condition, the output current is

automatically constrained at approximately

300mA. Continued current limit will cause internal

IC junction temperature increased. When the

temperature of device exceeds 150°C, the

thermal protection will shut the switching down

and the temperature will reduce afterwards. Once

the temperature drops below 135°C, the charge

pump switching circuit will re-start. If the fault

doesn’t eliminate, the above protecting operation

will repeat again and again. It allows AIC1848 to

continuously work at short circuit condition without

damaging the device.

Shutdown

In shutdown mode, the output is disconnected

from input. The input current gets extremely low

since most of the circuitry is turned off. Due to

high impedance, shutdown pin can’t be floated.

Efficiency

Refer to Fig. 12 and Fig. 13, they shows the circuit

of charge pump at different operation states.

RDS-ON is the resistance of the switching element

at conduction.

ESR is the equivalent series resistance of the

flying capacitor CFLY.

ION-AVE and IOFF-AVE are the average current

during on state and off state, respectively.

D is the duty cycle, which means the proportion

the on state takes.

Let’s take advantage of conversation of charge for

capacitor CFLY. Assume that the capacitor CFLY

has reached its steady state. The amount of

charge flowing into CFLY during on state is equal

to that flowing out of CFLY at off state.

ION−AVE × DT = IOFF−AVE × (1 − D)T

(1)

ION-AVE × D = IOFF-AVE × (1− D)

(2)

IIN = ION-AVE × D + IOFF-AVE × (1− D)

= 2 × ION-AVE × D

(3)

= 2 × IOFF-AVE × (1- D)

IOUT = IOFF-AVE × (1− D)

IIN = 2 × IOUT

(4)

For AIC1848, the controller takes the PWM (Pulse

Width Modulation) control strategy. When the duty

cycle is limited to 0.5, there will be:

ION-AVE × 0.5 × T = IOFF-AVE × (1− 0.5) × T

ION-AVE = IOFF-AVE

According to the equation (4), we know that as

long as the flying capacitor CFLY is at steady state,

input current is double of output current. The

efficiency of charge pump is given below:

η = VOUT × IOUT = VOUT × IOUT = VOUT

VIN × IIN

VIN × 2IOUT 2VIN

VIN

CIN

ION

RDS-ON-Q1 ESR Q2

CFLY

VOUT

COUT

Q3

RDS-ON-Q4

Fig. 12 The on state of charge pump circuit

7

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