© 2015 Exar Corporation

XR79110

12 / 20

exar.com/XR79110

Rev 2A

Functional Description

XR79110 is a synchronous step-down proprietary emulated

current-mode Constant On-Time (COT) Module. The on-

The emulated current-mode control is stable with ceramic

output capacitors.

Each switching cycle begins with GH signal turning on the

high-side (switching) FET for a preprogrammed time. At the

end of the on-time, the high-side FET is turned off and the

low-side (synchronous) FET is turned on for a preset mini-

mum time (250ns nominal). This parameter is termed Mini-

mum Off-Time. After the minimum off-time, the voltage at

the feedback pin FB is compared to an internal voltage

the ramp voltage, the high-side FET is turned on and the

cycle repeats. This voltage ramp constitutes an emulated

current ramp and makes possible the use of ceramic

capacitors, in addition to other capacitor types, for output fil-

tering.

Enable/Mode Input (EN/MODE)

EN/MODE pin accepts a tri-level signal that is used to con-

trol turn on/off. It also selects between two modes of opera-

tion: ‘Forced CCM’ and ‘DCM/CCM’. If EN is pulled below

1.8V, the Module shuts down. A voltage between 2.0V and

2.8V selects the Forced CCM mode which will run the Mod-

ule in continuous conduction at all times. A voltage higher

than 3.1V selects the DCM/CCM mode which will run the

Module in discontinuous conduction at light loads.

Selecting the Forced CCM Mode

In order to set the Module to operate in Forced CCM, a volt-

age between 2.0V and 2.8V must be applied to EN/MODE.

This can be achieved with an external control signal that

meets the above voltage requirement. Where an external

control is not available, the EN/MODE can be derived from

cuit shown in figure 25 can be used to generate the

nal Zever voltage is 3.8V and 4.7V respectively. Therefore

Selecting the DCM/CCM Mode

In order to set the Module operation to DCM/CCM, a volt-

age between 3.1V and 5.5V must be applied to EN/MODE

pin. If an external control signal is available, it can be

directly connected to EN/MODE. In applications where an

external control is not available, EN/MODE input can be

range, the circuit shown in figure 26 can be used to gener-

ate the required voltage.

Figure 25: Selecting Forced CCM by deriving EN/MODE from

Figure 26: Selecting DCM/CCM by deriving EN/MODE from

R1

30.1k, 1%

RZ

10k

EN/MODE

R2

35.7k, 1%

Zener

MMSZ4685T1G or Equivalent

IN

V

Zener

MMSZ4685T1G or Equivalent

RZ

10k

EN/MODE

V

IN

V

EN