LM3103 datasheet(13/20 Pages) TI1

13 / 20 page

tSS = 180 Ps +

70 PA

CSS x 0.6V

CIN =

'VIN

IOUT x tON

LM3103

www.ti.com

SNVS523F – SEPTEMBER 2007 – REVISED APRIL 2013

Figure 25. Inductor selection for VOUT = 0.6V

Figure 24 and Figure 25 show curves on inductor selection for various VOUT and RON. According to Equation 8,

VIN is limited for small RON. Some curves are therefore limited as shown in the figures.

CVCC: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false

triggering of the VCC UVLO at the main MOSFET on/off transitions. CVCC should be no smaller than 1 µF for

stability, and should be a good quality, low ESR, ceramic capacitor.

COUT and COUT3: COUT should generally be no smaller than 10 µF. Experimentation is usually necessary to

determine the minimum value for COUT, as the nature of the load may require a larger value. A load which

creates significant transients requires a larger COUT than a fixed load.

COUT3 is a small value ceramic capacitor located close to the LM3103 to further suppress high frequency noise at

VOUT. A 47 nF capacitor is recommended.

CIN and CIN3: The function of CIN is to supply most of the main MOSFET current during the on-time, and limit the

voltage ripple at the VIN pin, assuming that the voltage source connecting to the VIN pin has finite output

impedance. If the voltage source’s dynamic impedance is high (effectively a current source), CIN supplies the

difference between the instantaneous input current and the average input current.

At the maximum load current, when the main MOSFET turns on, the current to the VIN pin suddenly increases

from zero to the valley of the inductor’s ripple current and ramps up to the peak value. It then drops to zero at

turn-off. The average current during the on-time is the load current. For a worst case calculation, CIN must be

capable of supplying this average load current during the maximum on-time. CIN is calculated from:

(10)

where IOUT is the load current, ton is the maximum on-time, and ΔVIN is the allowable ripple voltage at VIN.

CIN3’s purpose is to help avoid transients and ringing due to long lead inductance at the VIN pin. A low ESR 0.1

µF ceramic chip capacitor located close to the LM3103 is recommended.

CBST: A 33 nF high quality ceramic capacitor with low ESR is recommended for CBST since it supplies a surge

current to charge the main MOSFET gate driver at each turn-on. Low ESR also helps ensure a complete

recharge during each off-time.

CSS: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the

regulation comparator and therefore, the output voltage to reach their final value. The time is determined from the

following equation:

(11)

CFB: If the output voltage is higher than 1.6V, CFB is needed in the Discontinuous Conduction Mode to reduce the

output ripple. The recommended value for CFB is 10 nF.

Submit Documentation Feedback

Product Folder Links: LM3103

LM3103 Datasheet(PDF) 13 Page - Texas Instruments

LM3103 Datasheet(HTML) 13 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL