A6986F datasheet(31/72 Pages) STMICROELECTRONICS

31 / 72 page

DocID027739 Rev 1

31/72

A6986F

Functional description

5.6

Switchover feature

The switchover maximizes the efficiency at the light-load that is crucial for LCM applications.

5.6.1

LCM

The LCM operation satisfies the high efficiency requirements of the battery powered

applications. In order to minimize the regulator quiescent current request from the input

voltage, the VBIAS pin can be connected to an external voltage source in the range

3 V < VBIAS < 5.5 V (see Section 5.1: Power supply and voltage reference on page 15).

In case the VBIAS pin is connected to the regulated output voltage (VOUT), the total current

drawn from the input voltage can be calculated as:

Equation 14

where IQ OP VIN, IQ OP VBIAS are defined in Table 5: Electrical characteristics on page 8

and

A6986 is the efficiency of the conversion in the working point.

5.6.2

LNM

Equation 14 is also valid when the device works in LNM and it can increase the efficiency at

the medium load since the regulator always operates in the continuous conduction mode.

5.7

Overcurrent protection

The current protection circuitry features a constant current protection, so the device limits

the maximum peak current (see Table 5) in overcurrent condition.

The A6986F device implements a pulse by pulse current sensing on both power elements

(high-side and low-side switches) for effective current protection over the duty cycle range.

The high-side current sensing is called “peak” the low-side sensing “valley”.

The internal noise generated during the switching activity makes the current sensing

circuitry ineffective for a minimum conduction time of the power element. This time is called

“masking time” because the information from the analog circuitry is masked by the logic to

prevent an erroneous detection of the overcurrent event. As a consequence, the peak

current protection is disabled for a masking time after the high-side switch is turned on, the

valley for a masking time after the low-side switch is turned on. In other words, the peak

current protection can be ineffective at extremely low duty cycles, the valley current

protection at extremely high duty cycles.

The A6986F device assures an effective overcurrent protection sensing the current flowing

in both power elements. In case one of the two current sensing circuitry is ineffective

because of the masking time, the device is protected sensing the current on the opposite

switch. Thus, the combination of the “peak” and “valley” current limits assure the

effectiveness of the overcurrent protection even in extreme duty cycle conditions.

The valley current threshold is designed higher than the peak to guarantee a proper

operation. In case the current diverges because of the high-side masking time, the low-side

power element is turned on until the switch current level drops below the valley current

IQVIN

IQOPVIN



A6986

-----------------

VBIAS

VIN

--------------- IQOPVBIAS



A6986F 数据表(PDF) 31 Page - STMicroelectronics

A6986F 数据表(HTML) 31 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

数据表下载