L6920D.PDF

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1V HIGH EFFICIENCY SYNCHRONOUS STEP UP CONVERTER
L6920
1V HIGH EFFICIENCY SYNCRONOUS STEP UP CONVERTER
1 Features
■ 0.6 TO 5.5V OPERATING INPUT VOLTAGE
■ 1V START UP INPUT VOLTAGE
■ INTERNAL SYNCHRONOUS RECTIFIER
■ ZERO SHUT DOWN CURRENT
■ 3.3V AND 5V FIXED OR ADJUSTABLE
OUTPUT VOLTAGE (2V UP TO 5.2V)
■ 120m
Figure 1. Package
TSSOP8
Table 1. Order Codes
Part Number
Package
INTERNAL ACTIVE SWITCH
■ LOW BATTERY VOLTAGE DETECTION
■
Ω
L6920D
TSSOP8 Tube
L6920DTR
Tape & Reel
REVERSE BATTERY PROTECTION
1.1 Applications
2 Description
The L6920 is a high efficiency step-up controller re-
quiring only three external components to realize the
conversion from the battery voltage to the selected
output voltage.
The start up is guaranteed at 1V and the device is op-
erating down to 0.6V.
Internal synchronous rectifier is implemented with a
120m
ONE TO THREE CELL BATTERY DEVICES
■ PDA AND HAND HELD INSTRUMENTS
■ CELLULAR PHONES - DIGITAL CORDLESS
PHONE
■ PAGERS
■ GPS
■
DIGITAL CAMERAS
P-channel MOSFET and, in order to improve
the efficiency, a variable frequency control is imple-
mented.
Ω
Figure 1. Application Circuit
L1
LX
OUT
V CC
V OUT
7
8
2.5V
C2
FB
3.3V
500mA
1
SHDN
5
L6920D
C3
C1
LBI
2
3
LBO
REF
4
6
GND
Rev. 2
February 2005
1/13
■
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L6920
Table 1. Pin Description
Pin
Name
Function
1
FB
Output voltage selector. Connect FB to GND for Vout=5V or to OUT for Vout=3.3V. Connect FB to an
external resistor divider for adjustable output voltage (from 2V to 5.2V) [see R4 and R5, fig. 7].
2
LBI
Battery low voltage detector input. The internal threshold is set to 1.23V.
A resistor divider is needed to adjust the desired low battery threshold:
V LBI
=
1.23V
⋅
+
⎝ ⎠
1
R1
R2
[see R1 and R2, fig. 7]
3
LBO Battery low voltage d etecto r output. If the voltage at the LBI pin drops below the internal
thre shold typ. 1.23V, LBO goes low.
The LBO is an open drain output and so a pull-up resistor (about 200K Ω ) has to be added for
correct output setting [see R3, fig. 7].
4
REF
1.23V reference voltage. Bypass this output to GND with a 100nF capacitor for filtering high
frequency noise. No capacitor is required for stability
5
SHDN Shutdown pin. When pin 5 is below 0.2V the device is in shutdown, when pin 5 is above 0.6V the
device is operating.
6
GND
Ground pin
7
LX
Step-up inductor connection
8
OUT
Power OUTPUT pin
Figure 2. Pin Connection (Top view)
FB
LBI
1
8
OUT
2
7
LX
LBO
3
6
5
GND
REF
4
SHDN
TSSOP8
Table 2. Absolute Maximum Ratings
Symbol
Parameter
Value
Unit
V ccmax
V cc to GND
6
V
LBI, SHDN, FB to GND
6
V
V out max
Vout to GND
6
V
Table 3. Thermal Data
Symbol
Parameter
Value
Unit
R th j-amb
Thermal Resistance Junction to Ambient
250
°C/W
T j
Maximum Junction Temperature
150
°C
2/13
⎛ ⎞
--------
647858104.071.png 647858104.072.png
L6920
Table 4. Electrical Characteristcs
(V in = 2V, FB = GND, T amb = -40°C to 85°C and T j < 125°C unless otherwise specified)
Symbol
Parameter
Test Condition
Min.
Typ.
Max.
Unit
V CC SECTION
V in
Minimum operating Input Voltage
0.6
V
V in
Minimum Start Up Input Voltage
1
V
I q
Quiescent Current
I l =0 mA, FB = 1.4V, V out = 3.3V
LBI = SHDN = 2V, T j = T amb
9
15
µ
A
I l =0 mA, FB = 1.4V, V out = 5V
LBI = SHDN = 2V, T j = T amb
11
18
µ
A
I sd
Shut Down Current
V in = 5V, I l =0 mA
0.1
1
µ
A
Irev
Reverse battery current
V in = -4V, T j = T amb
0.1
2
µ
A
POWER SECTION
R on-N Active switch ON resistance
120
250
m
Ω
R on-P
Synchronous switch ON
resistance
120
250
m
Ω
CONTROL SECTION
V ou t
Output voltage
FB = OUT, I l =0 mA
3.2
3.3
3.4
V
FB = GND, I l =0 A
4.9
5
5.1
V
Output voltage range
External divider
2
5.2
V
V LBI
LBI threshold
1.18
1.23
1.27
V
0°C < T j < 70°C
1.205
1.23
1.255
V
V LBO LBO logic LOW
I sink < 250
A
0.2
0.4
V
I lim
LX switch current limit
0.8
1
1.2
A
T onmax Maximum on time
V out = 2V to 5.3V
3.75
5
6.25
µ
s
T offmin Minimum off time
V out = 2V to 5.3V
0.75
1
1.25
µ
s
SHDN SHDN logic LOW
0.2
V
SHDN logic HIGH
0.6
V
V ref
Reference Voltage
1.18
1.23
1.27
V
3/13
µ
647858104.073.png
L6920
Figure 3. Efficiency vs. Output Current
Figure 5. Startup Voltage vs Output Current
100
1.4
Vin = 2.4V
90
1.2
80
Vin = 1.2V
70
1
60
0.8
50
40
0.6
30
Vout = 3.3V
L = 47
20
H
C = 100 µ F
µ
0.4
H
C = 22 µ F
µ
10
0.2
0
0.01
0.1
1
10
100
1000
LOAD CURRENT [mA]
0
30
60
90
120
150
180
Figure 4. Efficiency vs. Output Current
Output current (mA)
100
Vin = 3.6V
90
80
Vin = 2.4V
Vin = 1.2V
70
60
50
f
Vout = 5V
L = 47 µ H
C = 100 µ F
40
30
20
10
0
0.01
0.1
1
10
100
1000
LOAD CURRENT [mA]
4/13
L = 47
647858104.074.png
L6920
3 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
µ
Figure 6. Simplified Block Diagram
OUT
V O U T
ZERO CROSSING
-
+
V REF
-
+
- +
VBG
SHDN
FB
Y
VO U T
V OUT
GND
R 1 ,R 2
A
B
C
L X
V IN
A
B
Y
-
+
OPAMP
(CR)
C
VBG
GND
Toff min
1
sec
Q
S
R
-
+
CURRENT LIMIT
FB
LBO
-
+
Ton max
5
VBG
µ
sec
LBI
D99IN1041
4 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
µ
s. 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.
µ
So, in case of light load, the device works in PFM mode, as shown in figures 7 to 10.
5/13
A 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.
µ
s.
- 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
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