19-1290; Rev 1; 2/98
Regulated, Adjustable -2x
Inverting Charge Pump
MAX868
General Description
The MAX868 inverting charge pump provides a low-cost
and compact means of generating a regulated negative
voltage up to -2 x V
IN
from a positive input voltage
between 1.8V and 5.5V. It uses a pulse-frequency-
modulation (PFM) control scheme to generate the regulat-
ed negative output voltage. PFM operation is obtained
by gating the internal 450kHz oscillator on and off as
needed to maintain output voltage regulation. This
unique on-demand switching scheme gives the MAX868
excellent light-load efficiency without degrading its full-
load operation (up to 30mA), permitting smaller capaci-
tors to take advantage of the oscillator’s high switching
frequency.
The MAX868 requires no inductors; only four capacitors
are required to build a complete DC-DC converter.
Output voltage regulation is achieved by adding just two
resistors. The MAX868 comes in a 10-pin µMAX pack-
age, which is only 1.11mm high and occupies just half
the board area of a standard 8-pin SO.
____________________________Features
o
Regulated Negative Output Voltage
(up to -2 x V
IN
)
o
Ultra-Small, 10-Pin µMAX Package
o
On-Demand Switching at up to 450kHz
o
30µA Quiescent Supply Current
o
Requires Only Four Small External Capacitors
o
1.8V to 5.5V Input Voltage Range
o
0.1µA Logic-Controlled Shutdown
o
Up to 30mA Output Current
Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
Dice*
10 µMAX
MAX868C/D
0°C to +70°C
MAX868EUB
-40°C to +85°C
*Dice
are tested at T
A
= +25°C.
________________________Applications
Small LCD Panels
Cell Phones
Cordless Phones
Camcorders
Handy-Terminals, PDAs
Medical Instruments
Battery-Operated Equipment
Typical Operating Circuit
Configuration
V
IN
= 1.8V TO 5.5V
1µF
TOP VIEW
SHDN
IN
MAX868
GND
1
OUT
C1-
PGND
C1+
2
3
4
5
10
FB
9
SHDN
C2+
IN
C2-
C2+
0.1µF
C2-
PGND
GND
2.2µF
OUT
C1+
0.1µF
C1-
V
OUT
= 0V TO -2 x V
IN
FB
MAX868
8
7
6
µMAX
________________________________________________________________
Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
Regulated, Adjustable -2x
Inverting Charge Pump
MAX868
ABSOLUTE MAXIMUM RATINGS
IN to GND .................................................................-0.3V to +6V
OUT to GND ...........................................................+0.3V to -12V
IN to OUT.................................................................-0.3V to -17V
C1+ to GND ........................................(V
IN
- 12V) to (V
IN
+ 0.3V)
C1- to GND.............................................................+0.3V to -12V
C2+ to GND ....................................................(V
IN
+ 0.3V) to -6V
C2- to GND...............................................................+0.3V to -6V
SHDN,
FB to GND .......................................-0.3V to (V
IN
+ 0.3V)
PGND to GND .......................................................-0.3V to +0.3V
Output Current ....................................................................35mA
Short-Circuit Duration.................................................Continuous
Continuous Power Dissipation (T
A
= +70°C)
10-pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW
Operating Temperature Range
MAX868EUB ....................................................-40°C to +85°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
IN
= +3.3V,
SHDN
= IN, C1 = C2 = 0.22µF, C
IN
= 1µF, C
OUT
= 10µF,
T
A
= 0°C to +85°C,
unless otherwise noted. Typical values
are at T
A
= +25°C.)
PARAMETER
Supply-Voltage Range
Supply Current
Shutdown Current
Oscillator Frequency
Closed-Loop Output
Resistance
Open-Loop Output
Resistance
FB Trip Point
Output Current
FB Input Bias Current
SHDN
Input Threshold
SHDN
Input Bias Current
V
IL
V
IH
I
OUT
SYMBOL
V
IN
I
IN
I
IN
,
SHDN
f
OSC
R
OUT,CL
R
L
= 3kΩ to GND
No load, V
FB
= -50mV
FB = IN
No load,
SHDN
= GND
V
FB
= 50mV
V
OUT
= -5V
I
OUT
= 5mA, FB = IN
T
A
= +25°C
T
A
= 0°C to +85°C
15
-30
-40
12
30
-50
0.3V
IN
0.7V
IN
-100
1
100
1
50
T
A
= +25°C
T
A
= 0°C to +85°C
V
IN
= 3.3V, V
OUT
= -5V
V
IN
= 5V, V
OUT
= -3.3V
T
A
= +25°C
T
A
= 0°C to +85°C
293
270
0.2
70
100
125
50
30
40
mV
mV
mA
nA
V
nA
Ω
CONDITIONS
MIN
1.8
30
5
0.1
450
1
607
630
TYP
MAX
5.5
50
UNITS
V
µA
mA
µA
kHz
Ω
R
OUT
SHDN
= GND (OUT pulls to GND)
V
IN
= 1.8V to 5.5V
Closed loop
V
IN
= 1.8V to 5.5V, T
A
= +25°C
V
IN
= 1.8V to 5.5V
V
IN
= 5.5V,
SHDN
= IN or GND
2
_______________________________________________________________________________________
Regulated, Adjustable -2x
Inverting Charge Pump
ELECTRICAL CHARACTERISTICS
(V
IN
= +3.3V, C1 = C2 = 0.22µF, C
IN
= 1µF, C
OUT
= 10µF,
T
A
= -40°C to +85°C,
unless otherwise noted. (Note 1)
PARAMETER
Supply-Voltage Range
Supply Current
Shutdown Current
Oscillator Frequency
Open-Loop Output
Resistance
FB Trip Point
FB Input Bias Current
SHDN
Input Threshold
SHDN
Input Bias Current
V
IL
V
IH
SYMBOL
V
IN
I
IN
I
IN,SHDN
f
OSC
R
OUT
R
L
= 3kΩ to GND
No load, V
FB
= -50mV
No load,
SHDN
= GND
V
FB
= 50mV
I
OUT
= 5mA, FB = IN
SHDN
= GND (OUT pulls to GND)
V
IN
= 1.8V to 5.5V
V
IN
= 1.8V to 5.5V
V
IN
= 1.8V to 5.5V
V
IN
= 5.5V,
SHDN
= IN or GND
-40
-100
0.3V
IN
0.7V
IN
-100
100
270
CONDITIONS
MIN
1.8
TYP
MAX
5.5
55
1
630
125
50
40
100
UNITS
V
µA
µA
kHz
Ω
mV
nA
V
nA
MAX868
Note 1:
Specifications to -40°C are guaranteed by design, not production tested.
__________________________________________Typical Operating Characteristics
(Circuit of Figure 5, T
A
= +25°C, unless otherwise noted.)
LOAD-REGULATION ERROR
vs. LOAD CURRENT
(V
IN
= 5V)
MAX868-01
LOAD-REGULATION ERROR
vs. LOAD CURRENT
(V
IN
= 3.3V)
MAX868-02
MAXIMUM SWITCHING FREQUENCY
vs. TEMPERATURE
MAXIMUM SWITCHING FREQUENCY (kHz)
490
480
470
460
450
440
430
420
410
400
-40
-20
V
IN
= 2V
0
20
40
60
80
100
V
IN
= 3.3V
V
IN
= 5V
FB = IN
MAX868-03
5
LOAD-REGULATION ERROR (mV)
0
-5
-10
V
OUT
= -7.5V
-15
-20
V
OUT
= -3.3V
-25
-30
-35
0
5
V
OUT
= -5V
3
LOAD-REGULATION ERROR (mV)
0
-3
-6
-9
-12
-15
V
OUT
= -5V
500
V
OUT
= -3.3V
10 15 20 25 30 35 40 45 50
LOAD CURRENT (mA)
0
5
10
15
20
25
LOAD CURRENT (mA)
TEMPERATURE (°C)
_______________________________________________________________________________________
3
Regulated, Adjustable -2x
Inverting Charge Pump
MAX868
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 5, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(V
IN
= 5V)
MAX868-04
EFFICIENCY vs. LOAD CURRENT
(V
IN
= 3.3V)
MAX868-05
EFFICIENCY vs. LOAD CURRENT
(V
IN
= 5V)
CIRCUIT OF FIGURE 6
70
60
EFFICIENCY (%)
50
40
30
20
10
0
V
OUT
= -2.5V
V
OUT
= -3.3V
MAX868-06
80
70
60
EFFICIENCY (%)
50
40
30
20
10
0
0.01
0.1
1
LOAD CURRENT (mA)
10
V
OUT
= -3.3V
V
OUT
= -5V
V
OUT
= -7.5V
80
70
V
OUT
= -5V
60
EFFICIENCY (%)
50
40
30
20
10
0
V
OUT
= -3.3V
80
100
0.01
0.1
1
LOAD CURRENT (mA)
10
100
0.01
0.1
1
LOAD CURRENT (mA)
10
100
OPEN-LOOP OUTPUT IMPEDANCE
vs. TEMPERATURE
(FB = IN, V
OUT
= -2 x V
IN
)
MAX868-07
OPEN-LOOP OUTPUT IMPEDANCE
vs. TEMPERATURE
(FB = IN, V
OUT
= -V
IN
)
MAX868-08
LOAD-TRANSIENT RESPONSE
MAX868-12
200
180
OUTPUT IMPEDANCE (Ω)
160
140
120
100
80
60
40
20
0
-40
-20
0
20
40
60
80
V
IN
= 5V
V
IN
= 2V
V
IN
= 3.3V
200
180
OUTPUT IMPEDANCE (Ω)
160
140
120
100
80
60
40
20
0
V
IN
= 5V
-40
-20
0
20
40
60
80
V
IN
= 3.3V
V
IN
= 2V
CIRCUIT OF FIGURE 6
10mA/div
20mV/div
100
100
TEMPERATURE (°C)
TEMPERATURE (°C)
200µs/div
V
IN
= 5V, V
OUT
= -5V, I
OUT
= 1mA TO 11mA STEP
OUTPUT VOLTAGE RIPPLE
(C
OUT
= 10µF TANTALUM)
MAX868-09
OUTPUT VOLTAGE RIPPLE
(C
OUT
= 10µF CERAMIC)
MAX868-10
OUTPUT VOLTAGE RIPPLE
MAX868-11
20mV/div
20mV/div
20mV/div
20µs/div
V
IN
= 3.3V, V
OUT
= -3.3V, I
LOAD
= 5mA,
V
OUT
AC COUPLED (20mV/div), C
OUT
= 10µF (AVX TPS)
20µs/div
V
IN
= 3.3V, V
OUT
= -3.3V, I
LOAD
= 5mA,
V
OUT
AC COUPLED (20mV/div), C
OUT
= 10µF CERAMIC
20µs/div
V
IN
= 3.3V, V
OUT
= -3.3V, I
LOAD
= 5mA,
V
OUT
AC COUPLED (20mV/div), C
OUT
= 2.2µF CERAMIC
4
_______________________________________________________________________________________
Regulated, Adjustable -2x
Inverting Charge Pump
Pin Description
PIN
1
2
3
4
5
6
7
8
9
10
NAME
GND
OUT
C1-
PGND
C1+
C2-
IN
C2+
SHDN
FB
Analog Ground
Charge-Pump Output
Negative Terminal of Flying Capacitor C1
Power Ground
Positive Terminal of Flying Capacitor C1
Negative Terminal of Flying Capacitor C2
Supply-Voltage Input. Input voltage range is 1.8V to 5.5V.
Positive Terminal of Flying Capacitor C2
Active-Low Shutdown Input. Connect
SHDN
to GND to put the MAX868 in shutdown mode and reduce sup-
ply current to 0.1µA. Connect to IN for normal operation. OUT is actively pulled to GND in shutdown.
Feedback Input. Connect FB to a resistor divider for a regulated output voltage. Connect to IN to generate
an unregulated -2 x V
IN
output voltage.
FUNCTION
MAX868
Detailed Description
The MAX868 inverting charge pump uses pulse-
frequency-modulation (PFM) control to generate a reg-
ulated negative output voltage up to -2 x V
IN
. PFM
operation is obtained by enabling the internal 450kHz
oscillator as needed to maintain output voltage regula-
tion. This control scheme reduces supply current at
light loads and permits the use of small capacitors.
The functional diagram shown in Figure 1 indicates the
two phases of MAX868 operation: charge phase (Φ1)
and discharge phase (Φ2). In charge phase, the
switches on the left-hand side close, and the switches
on the right-hand side open. In the discharge phase,
the inverse occurs.
Figure 2 illustrates that in charge phase, both flying
capacitors are charged in parallel. The load is serviced
entirely by the charge stored in the output capacitor.
Figure 3 demonstrates the series connection of the fly-
ing capacitors in the discharge phase. The series com-
bination of the flying capacitors, when connected to the
output capacitor, transfers charge to the output in order
to maintain output voltage regulation. In normal opera-
tion, the MAX868 operates predominantly in charge
phase, switching to discharge phase only as needed to
maintain a regulated output.
C2+
IN
C2-
C1+
OUT
C1-
Φ1
SHDN
Φ2
FB
OSCILLATOR
C
OUT
V
REF
Figure 1. Functional Diagram
_______________________________________________________________________________________
5
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