LTC3534 datasheet(13/20 Pages) LINER

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LTC3534

3534fb

For more information www.linear.com/LTC3534

applicaTions inForMaTion

Output Capacitor Selection

The bulk value of the output filter capacitor is set to reduce

the ripple due to charge into the capacitor each cycle. The

steady state ripple due to charge is given by:

∆VP-PBoost =

IOUT • VOUT – VIN(MIN)

(

)

COUT • VOUT • f

∆VP-PBuck =

8 •L •COUT • f2

•

VIN(MAX) – VOUT

(

)•V

OUT

VIN(MAX)

where f = switching frequency in Hz, typically 1MHz.

COUT = output filter capacitor, F

IOUT = output load current, A

The output capacitance is usually many times larger than

theminimumvalueinordertohandlethetransientresponse

requirements of the converter. As a rule of thumb, the ratio

of the operating frequency to the unity-gain bandwidth of

the converter is the amount the output capacitance will

have to increase from the above calculations in order to

maintain the desired transient response. A 22µF or larger

ceramic capacitor is appropriate for most applications.

The other component of ripple is due to the ESR (equiva-

lent series resistance) of the output capacitor. Low ESR

capacitors should be used to minimize output voltage

ripple. For surface mount applications, Taiyo Yuden or

TDK ceramic capacitors, AVX TPS series tantalum capaci-

tors or Sanyo POSCAP are recommended. See Table 2 for

contact information.

Input Capacitor Selection

Since VIN is the supply voltage for the IC, as well as the

input to the power stage of the converter, it is recom-

mended to place at least a 10µF, low ESR ceramic bypass

capacitor close to the PVIN/VIN and PGND/GND pins. It is

also important to minimize any stray resistance from the

converter to the battery or other power source.

Optional Schottky Diodes

Schottky diodes across the synchronous switches B and

D are not required, but do provide a lower drop during the

break-before-make time (typically 15ns), thus improving

efficiency. Use a surface mount Schottky diode such as an

MBRM120T3 or equivalent. Do not use ordinary rectifier

diodes since their slow recovery times will compromise

efficiency.

Output Voltage < 1.8V

The LTC3534 can operate as a buck converter with output

voltages as low as 400mV. Since synchronous switch

D is powered from VOUT and the RDS(ON) will increase

significantly at output voltages below 1.8V typical, a

Schottky diode is required from SW2 to VOUT to provide

the conduction path to the output at low VOUT voltages.

The current limit is folded back to 800mA when VOUT <

0.9V typical which will significantly reduce the output

current capability of the application. Note that Burst Mode

operationisinhibitedatoutputvoltagesbelow1.6Vtypical.

Closing the Feedback Loop

TheLTC3534incorporatesvoltagemodePWMcontrol.The

control to output gain varies with operation region (buck,

boost, buck-boost), but is usually no greater than 15. The

output filter exhibits a double pole response, as given by:

fFILTER_POLE =

2•

π • L1•COUT

Hz (in buck mode)

fFILTER_POLE =

VIN

2• VOUT • π• L1•COUT

Hz (in boost mode)

where L1 is in Henries and COUT is in Farads.

The output filter zero is given by:

Table 2. Capacitor Vendor Information

SUPPLIER PHONE

FAX

WEBSITE

AVX

(803) 448-9411 (803) 448-1943 www.avxcorp.com

Sanyo

(619) 661-6322 (619) 661-1055 www.sanyovideo.com

Taiyo

Yuden

(408) 573-4150 (408) 573-4159 www.t-yuden.com

TDK

(847) 803-6100 (847) 803-6296 www.component.tdk.com

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