LTC3556_15 datasheet(25/36 Pages) LINER

25 / 36 page

LTC3556

3556f

Buck-Boost Regulator Soft-Start Operation

Soft-start is accomplished by gradually increasing the

reference voltage input to the error ampliﬁer over a 0.5ms

(typical) period. This limits transient inrush currents during

start-up because the output voltage is always “in regula-

tion.” Ramping the reference voltage input also limits the

rate of increase in the VC3 voltage which helps minimize

output overshoot during start-up. A soft-start cycle oc-

curs whenever the buck-boost is enabled, or after a fault

condition has occurred (thermal shutdown or UVLO). A

soft-start cycle is not triggered by changing operating

modes. This allows seamless operation when transitioning

between Burst Mode operation and PWM mode.

Low Supply Operation

The LTC3556 incorporates an undervoltage lockout circuit

on VOUT (connected to VIN3) which shuts down the buck-

boost regulator when VOUT drops below 2.6V. This UVLO

prevents unstable operation.

OPERATION

APPLICATIONS INFORMATION

CLPROG Resistor and Capacitor

As described in the High Efﬁciency Switching PowerPath

Controller section, the resistor on the CLPROG pin deter-

mines the average input current limit when the switching

regulator is set to either the 1x mode (USB 100mA), the

5x mode (USB 500mA) or the 10x mode. The input cur-

rent will be comprised of two components, the current

that is used to drive VOUT and the quiescent current of the

switching regulator. To ensure that the USB speciﬁcation

is strictly met, both components of input current should

be considered. The Electrical Characteristics table gives

values for quiescent currents in either setting as well as

current limit programming accuracy. To get as close to

the 500mA or 100mA speciﬁcations as possible, a 1%

resistor should be used. Recall that IVBUS = IVBUSQ +

VCLPROG/RCLPPROG • (hCLPROG +1).

An averaging capacitor or an R-C combination is required

in parallel with the CLPROG resistor so that the switching

regulator can determine the average input current. This

network also provides the dominant pole for the feedback

loop when current limit is reached. To ensure stability,

the capacitor on CLPROG should be 0.47μF or larger.

Alternatively, faster transient response may be achieved

with 0.1μF in series with 8.2Ω.

Choosing the PowerPath Inductor

Because the input voltage range and output voltage range

of the power path switching regulator are both fairly nar-

row, the LTC3556 was designed for a speciﬁc inductance

value of 3.3μH. Some inductors which may be suitable

for this application are listed in Table 6.

Table 6. Recommended Inductors for PowerPath Controller

INDUCTOR

TYPE

(μH)

MAX

IDC

(A)

MAX

DCR

(Ω)

SIZE IN mm

× W × H) MANUFACTURER

LPS4018

3.3

2.2

0.08

3.9

× 3.9 × 1.7 Coilcraft

www.coilcraft.com

D53LC

DB318C

3.3

2.26

1.55

0.034

0.070

5.0

× 5.0 × 3.0

3.8

× 3.8 × 1.8

Toko

www.toko.com

WE-TPC

Type M1

3.3

1.95 0.065 4.8

× 4.8 × 1.8 Würth Elektronik

www.we-online.com

CDRH6D12

CDRH6D38

3.3

2.2

3.5

0.0625

0.020

6.7

× 6.7 × 1.5

7.0

× 7.0 × 4.0

Sumida

www.sumida.com

VBUS and VOUT Bypass Capacitors

The style and value of capacitors used with the LTC3556

determine several important parameters such as regulator

control-loop stability and input voltage ripple. Because

the LTC3556 uses a step-down switching power supply

from VBUS to VOUT, its input current waveform contains

high frequency components. It is strongly recommended

that a low equivalent series resistance (ESR) multilayer

ceramic capacitor be used to bypass VBUS. Tantalum and

aluminum capacitors are not recommended because of

their high ESR. The value of the capacitor on VBUS directly

controls the amount of input ripple for a given load cur-

rent. Increasing the size of this capacitor will reduce the

input ripple.

LTC3556 датащи(PDF) 25 Page - Linear Technology

LTC3556 датащи(HTML) 25 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

датащи скачать

ссылки URL