Data Sheet

LT83201

analog.com

Rev. A 25 of 45

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

(~0.15V and ~0.15V, 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 maximum operating voltage of 18V regardless of the RT value;

however, LT83201 reduces switching frequency as necessary to maintain control of the inductor current to assure

safe operation.

In DCM, the LT83201 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 circuitry to

function correctly. In this mode, the LT83201 skips switch cycles, resulting in a lower switching frequency than

programmed by RT. The LT83201 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 LT83201 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 (~0.15V and ~0.15V, respectively, at maximum load), fSW is the switching frequency (set by

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

Inductor Selection and Maximum Output Current

The LT83201 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 LT83201 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

) × 0.3

(4)

where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.15V),

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.

In addition, the saturation current rating (typically labeled ISAT) of the inductor must be higher than the load current

plus ½ of the inductor ripple current. See Equation 5.

IL(PEAK) = ILOAD(MAX) +

1

2

∆IL

(5)