Data Sheet

LT7200S

analog.com

Rev 0

22 of 35

(b) Ratiometric Tracking

Figure 35. Output Tracking Configuration

Minimum Off-Time and Minimum On-Time Considerations

The minimum off-time, tOFF(MIN), is the smallest amount of time the LT7200S is capable of turning on the bottom power

MOSFET, tripping the current comparator and turning the power MOSFET back off. This time is generally about 40ns.

The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is

reached, due to a dropping input voltage for example, then the output drops out of regulation. The minimum input

voltage to avoid dropout is:

VIN(MIN) = VOUT x

tON + tOFF(MIN)

tON

Conversely, the minimum on-time is the smallest duration of time in which the top power MOSFET can be in its “on”

state. This time is typically 12ns. In continuous mode operation, the minimum on-time limit imposes a minimum

duty cycle of

DCMIN = f x tON(MIN)

where tON(MIN) is the minimum on-time. Reducing the operating frequency alleviates the minimum duty cycle

constraint.

In the rare cases where the minimum duty cycle is surpassed, the output voltage remains in regulation and the

switching frequency decreases from its programmed value. This is an acceptable result in many applications. So, this

constraint may not be of critical importance in most cases. High switching frequencies may be used in the design

without any fear of output overvoltage. As the sections on inductors and capacitor selection show, high switching

frequencies allow the use of smaller board components, thus reducing the size of the application circuit.

Internal/External ITH Compensation

The LT7200S provides the options to choose between internal and external loop compensations. By connecting the

ITH pin to INTVCC, the fixed internal loop compensation network is selected, which reduces both the required external

component count and design time. To ensure stability, it is recommended that internal compensation only be used

for applications with fSW > 1MHz.

Alternatively, by connecting the external components to the ITH pin, choose external loop compensation to optimize

the desired transient response. For multiphase, single output applications, use external compensation to tie the

corresponding ITH pins together to accurately share the output current.

VFB4

VOUT3

VOUT4

VOUT1

R2

R1

R8

R7

VOUT2

R4

R3

R6

R5

VFB1

TO

TRACK2,

TRACK3,

TRACK4

VFB2

VFB2