L6920DC

Detailed description

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Detailed description

The L6920DC 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 synchronous 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.

A simplified block diagram is shown here below.

Figure 5.

Simplified block diagram

5.1

Principle of operation

In L6920DC 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 L6920DC

directs the energy stored in the inductor to be transferred to the load. This is accomplished

by alternating between two basic steps:

●

node to ground via the N-channel power switch. The switch is turned off if the current

flowing in the inductor reaches the current limit or after a maximum on time set to 5

µs.

●

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.

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

- +