L6920DC Detailed description
7/16
5 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 (10µA 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:
● T
ON
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 the current limit or after a maximum on time set to 5µs.
● T
OFF
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 1µs. 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
5µsec
Toff min
1µsec
VBG
VBG
VBG
A
B
C
-
+
-
+
-
+
-
+
-
+
VOUT
ZERO CROSSING
OPAMP
(CR)
CURRENT LIMIT
OUT
FB
GND
V
REF
LBI
LBO
D99IN1041
SHDN
LX
V
IN
V
OUT
V
OUT
GND
R
1
,R
2
FB Y
Y
A
B
C
- +
