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L6920

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Detailed Description

The L6920 is a high efficiency, low voltage step-up DC/DC converter particularly suitable for 1 to 3 cells (Li-Ion/

polymer, NiMH respectively) battery up conversion.

These performances are achieved via a strong reduction of quiescent current (10

µA only) and adopting a syn-

chronous rectification, that implies also a reduced cost in the application (no external diode required).

Operation is based on maximum ON time - minimum OFF time control, tailored by a current limit set to 1A. A

simplified block diagram is shown here below.

Figure 6. Simplified Block Diagram

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Principle of Operation

In L6920 the control is based on a comparator that continuously checks the status of output voltage.

If the output voltage is lower than the expected value, the control function of the L6920 directs the energy stored

in the inductor to be transferred to the load. This is accomplished by alternating between two basic steps:

- TON phase: the energy is transferred from the battery to the inductor by shorting LX node to ground via the N-

channel power switch. The switch is turned off if the current flowing in the inductor reaches 1A or after a max-

imum on time set to 5

µs.

- TOFF phase: the energy stored in the inductor is transferred to the load through the synchronous switch for at

least a minimum off time equal to 1

µs. After this, the synchronous switch is turned off as soon as the output

voltage goes lower than the regulated voltage or the current flowing in the inductor goes down to zero.

So, in case of light load, the device works in PFM mode, as shown in figures 7 to 10.

R

Q

S

Ton max

5

µsec

Toff min

1

µsec

VBG

VBG

VBG

A

B

C

-

+

-

+

-

+

-

+

-

+

VOUT

ZERO CROSSING

OPAMP

(CR)

CURRENT LIMIT

OUT

FB

GND

LBI

LBO

D99IN1041

SHDN

LX

GND

FB

Y

Y

A

B

C

- +