Data Sheet

LT83203/LT83205

analog.com

Rev. B

25 of 44

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. The

advantage of high-frequency operation is that smaller inductor and capacitor values may be used. The

disadvantages are lower efficiency and a smaller input voltage range. The highest switching frequency (fSW(MAX)) for a

given application can be calculated by Equation 2.

fSW(MAX) =

VOUT + VSW(BOT)

tON(MIN) (VIN−VSW(TOP)+VSW(BOT))

(2)

where VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops

(~0.38V and ~0.11V, respectively, at maximum load), and tON(MIN) is the minimum top switch on-time (see the Electrical

Characteristics table). This equation shows that a slower switching frequency is necessary to accommodate a high

VIN/VOUT ratio.

For transient operation, VIN may go as high as the absolute maximum rating of 18V regardless of the RT value;

however, the LT83203/LT83205 reduces switching frequency as necessary to maintain control of the inductor current

to assure safe operation.

In pulse-skipping mode, the LT83203/LT83205 is capable of a maximum duty cycle of approximately 99%, and the

VIN-to-VOUT dropout is limited by the RDS(ON) of the top switch, provided there is sufficient headroom (~0.7V) between

VIN and SET for the current reference circuit to function correctly. In this mode, the LT83203/LT83205 skips switch

cycles, resulting in a lower switching frequency than programmed by RT. The LT83203/LT83205 switches as

frequently as necessary to keep the boost capacitor refreshed, with a minimum switching frequency of

approximately 80kHz. Note that higher switching frequencies increase the minimum input voltage below which

cycles are dropped to achieve a higher duty cycle.

In FCM, the LT83203/LT83205 does not skip cycles, and so the maximum duty cycle is limited by the minimum off-

time and chosen switching frequency. For applications that cannot allow deviation from the programmed switching

frequency at low VIN/VOUT ratios and thus must operate in FCM, use Equation 3 to set the switching frequency.

VIN(MIN) =

VOUT + VSW(BOT)

1 − fSW × tOFF(MIN)

− VSW(BOT) + VSW(TOP)

(3)

where VIN(MIN) is the minimum input voltage without skipped cycles, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are

the internal switch drops (~0.32V/0.54V and ~0.10V/0.16V, respectively, at maximum loads), fSW is the switching

frequency (set by RT), and tOFF(MIN) is the minimum switch off-time.

Inductor Selection and Maximum Output Current

The LT83203/LT83205 is designed to minimize solution size by allowing the inductor to be chosen based on the

output load requirements of the application. During overload or short-circuit conditions, the LT83203/LT83205 safely

tolerates operation with a saturated inductor through the use of a high-speed peak-current mode architecture.

A good starting point for the inductor value is given by Equation 4.

L = (

VOUT + VSW(BOT)

fSW

) × 0.8

(4)

where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop

(~0.11V/0.16V), and L is the inductor value in μH.

To avoid overheating and poor efficiency, choose an inductor with an RMS current rating that is greater than the

maximum expected output load of the application.