LTC3619BIDD(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LTC3619BIDD
(Rev.:RevA)

400mA/800mA Synchronous Step-Down DC/DC with Average Input Current Limit

Linear Technology 

LTC3619B

Operation

Short-Circuit Protection

When either regulator output is shorted to ground, the

corresponding internal N-channel switch is forced on for

a longer time period for each cycle in order to allow the

inductor to discharge, thus preventing inductor current

runaway. This technique has the effect of decreasing

switching frequency. Once the short is removed, normal

operation resumes and the regulator output will return to

its nominal voltage.

Input Current Limit

Internal current sense circuitry in each channel measures

the inductor current through the voltage drop across the

power PFET switch and forces the same voltage across

the small sense PFET. The voltage across the small sense

PFET generates a current representing 1/55,000th of the

inductor current during the on-cycle. The current out of

RLIM pin is the summed representation of the inductor

currents from both channels, which can be expressed in

the following equation.

IRLIM = IOUT1 • D1 • K1 + IOUT2 • D2 • K2,

where D1 = VOUT1/VIN and D2 = VOUT2/VIN are the duty

cycle of channel 1 and 2, respectively.

K1 is the ratio RDS(ON) (power PFET)/RDS(ON)(sense PFET)

of channel 1, and K2 is the ratio RDS(ON)(power PFET)/

RDS(ON) (sense PFET) of channel 2. The ratio of the power

PFET to the sense PFET is trimmed to within 2%.

Given that both PFETs are carefully laid out and matched,

their temperature and voltage coefficient effects will be

similar and their terms be canceled out in the equation. In

that case, the constants K1 and K2 will only be dependent

on area scaling, which is trimmed to within 2%. Thus, the

IRLIM current will track the input current very well over

varying temperature and VIN.

The RLIM pin can be grounded to disable input current

limit function.

Programming Input Current Limit

Selection of one external RLIM resistor will program the

input current limit. The current limit can be programmed

from 200mA up to IPEAK current. As the input current

increases, RLIM voltage will follow. When RLIM reaches

the internal comparator threshold of 1V, channel 2’s

power PFET on-time will be shortened, thereby, limiting

the input current.

Use the following equation to select the RLIM resistance

that corresponds to the input current limit.

RLIM

=

55kΩ − A

IDC(A)

IDC is the input current (at VIN) to be limited. The follow-

ing are some RLIM values with the corresponding current

limit.

RLIM

91.6k

110k

137.5k

IDC

600mA

500mA

400mA

Selection of CLIM Capacitance

Since IRLIM current is a function of the inductor current,

its dependency on the duty cycle cannot be ignored. Thus,

a CLIM capacitor is needed to integrate the IRLIM current

and smooth out transient currents. The LTC3619B is stable

with any size capacitance >100pF at the RLIM pin.

Each application input current limit will call for different

CLIM value to optimize its response time. Using a large CLIM

capacitor requires longer time for the RLIM pin voltage to

charge. For example, consider the application 500mA input

current limit, 5V input and 1A, 2.5V output with a 50% duty

cycle. When an instantaneous 1A output pulse is applied,

the current out of the RLIM pin becomes 1A/55k = 18.2µA

during the 50% on-time or 9.1µA full duty cycle. With a

CLIM capacitor of 1µF, RLIM of 116k, and using I = CdV/dt,

it will take 110ms for CLIM to charge from 0V to 1V. This is

the time after which the LTC3619B will start input current

3619bfa



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