19-1219; Rev 1; 6/97
AN
N KIT M
LUATIO ABLE
EVA
AVAIL
UAL
28V, Low-Power, High-Voltage,
Boost or Inverting DC-DC Converter
____________________________Features
o
Internal, 500mA, 28V N-Channel Switch
(No External FET Required)
o
Generates Positive or Negative Output Voltages
o
80µA Supply Current
o
1µA Max Shutdown Current
o
Up to 300kHz Switching Frequency
o
Adjustable Current Limit Allows Use of Small,
Inexpensive Inductors
o
8-Pin SO Package
_______________General Description
The MAX629 low-power DC-DC converter features an
internal N-channel MOSFET switch and programmable
current limiting. It is designed to supply positive or neg-
ative bias voltages up to ±28V from input voltages in
the 0.8V to V
OUT
range, and can be configured for
boost, flyback, and SEPIC topologies.
The MAX629’s current-limited pulse-frequency-modula-
tion (PFM) control scheme provides high efficiency over
a wide range of load conditions. An internal, 0.5A N-
channel MOSFET switch reduces the total part count,
and a high switching frequency (up to 300kHz) allows
for tiny surface-mount magnetics.
The MAX629’s combination of low supply current, logic-
controlled shutdown, small package, and tiny external
components makes it an extremely compact and effi-
cient high-voltage biasing solution that’s ideal for bat-
tery-powered applications. The MAX629 is available in
an 8-pin SO package.
MAX629
______________Ordering Information
PART
MAX629C/D
MAX629ESA
TEMP. RANGE
0°C to +70°C
-40°C to +85°C
PIN-PACKAGE
Dice*
8 SO
________________________Applications
Positive or Negative LCD Bias Generators
High-Efficiency DC-DC Boost Converters
Varactor Tuning Diode Bias
Palmtop Computers
2-Cell and 3-Cell Battery-Powered Applications
*Dice
are tested at T
A
= +25°C, DC parameters only.
Note:
To order tape-and-reel shipping, add “-T” to the end of
the part number.
Pin Configuration appears at end of data sheet.
___________________________________________________Typical Operating Circuit
V
IN
+2.7V
TO +5.5V
V
OUT
28V
V
IN
+2.7V
TO +5.5V
V
CC
SHDN
ISET
POL
FB
LX
V
CC
SHDN
ISET
POL
LX
-V
OUT
-28V
MAX629
REF
GND
MAX629
FB
REF
GND
POSITIVE OUTPUT VOLTAGE
NEGATIVE OUTPUT VOLTAGE
________________________________________________________________
Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
28V, Low-Power, High-Voltage,
Boost or Inverting DC-DC Converter
MAX629
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V
CC
to GND) ..................................-0.3V to +6V
SHDN
to GND...........................................................-0.3V to +6V
ISET, REF, FB, POL to GND .......................-0.3V to (V
CC
+ 0.3V)
LX to GND ..............................................................-0.3V to +30V
Continuous Power Dissipation (T
A
= +70°C)
SO (derate 5.88mW/°C above +70°C) ..........................471mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +165°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
CC
= +5V, C
REF
= 0.1µF, T
A
= -40°C to +85°C, unless otherwise noted. Typical values are at T
A
= +25°C.) (Note1)
PARAMETER
V
CC
Input Voltage (Note 2)
V
CC
Supply Current
V
CC
Shutdown Current
V
CC
Undervoltage Lockout
Input Supply Voltage (Note 2)
SHDN,
POL, ISET Logic Levels
Positive Output Voltage
Negative Output Voltage
LX Switch-Current Limit
LX On-Resistance
LX Leakage Current
Maximum LX On-Time
POL = GND
Minimum LX Off-Time
POL = V
CC
POL = GND, V
FB
< 1V
POL = V
CC,
V
FB
> 0.25V
POL = GND
(positive output)
FB Set Point
POL = V
CC
(negative output)
FB Input Bias Current
REF Output Voltage
V
CC
= 2.7V to 5.5V,
no load on REF
T
A
= 0°C to +85°C
T
A
= -40°C to +85°C
1.225
1.218
T
A
= 0°C to +85°C
T
A
= -40°C to +85°C
T
A
= 0°C to +85°C
T
A
= -40°C to +85°C
V
FB
= 1.3V
SHDN
= GND
100mV hysteresis
Voltage applied to L1 (V
IN
)
V
IH
V
IL
Circuit of Figure 2
Circuit of Figure 3
ISET = V
CC
ISET = GND
V
CC
= 5V
V
CC
= 3.3V
V
LX
= 28V, T
A
= +85°C
6.5
0.7
2.0
3.0
3.0
1.225
1.218
-15
-25
5
1.250
0
-
V
IN
0.39
0.20
0.45
0.25
0.6
0.7
0.05
8.5
1.0
3.2
4.5
4.5
1.250
2.3
0.8
2.4
0.4
28
-28
0.51
0.33
1.2
1.4
2.5
10.0
1.3
3.8
6.0
6.0
1.275
1.282
15
25
50
1.275
1.282
V
mV
nA
V
µs
CONDITIONS
MIN
2.7
80
0.04
2.5
TYP
MAX
5.5
120
1
2.65
V
OUT
UNITS
V
µA
µA
V
V
V
V
V
A
Ω
µA
µs
2
_______________________________________________________________________________________
28V, Low-Power, High-Voltage,
Boost or Inverting DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V
CC
= +5V, C
REF
= 0.1µF, T
A
= -40°C to +85°C, unless otherwise noted. Typical values are at T
A
= +25°C.) (Note 1)
PARAMETER
REF Load Regulation
Line Regulation
Load Regulation
Thermal Shutdown Threshold
CONDITIONS
I
REF
= 0µA to 100µA, C
REF
= 0.47µF (Note 3)
Circuit of Figure 2, V
OUT
= 24V, V
CC
= 3V to 5.5V,
I
LOAD
= 5mA
Circuit of Figure 2, V
OUT
= 24V, V
CC
= 5V,
I
LOAD
= 0mA to 5mA
Die temperature
MIN
TYP
10
0.2
0.15
150
MAX
25
UNITS
mV
%/V
%
°C
MAX629
Note 1:
Specifications to -40°C are guaranteed by design and not production tested.
Note 2:
The IC itself requires a supply voltage between +2.7V and +5.5V; however, the voltage that supplies power to the inductor
can vary from 0.8V to 28V, depending on circuit operating conditions.
Note 3:
For reference currents less than 10µA, a 0.1µF reference-bypass capacitor is adequate.
__________________________________________Typical Operating Characteristics
(SHDN = V
CC
, C
REF
= 0.1µF, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= +24V)
MAX629-01
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= +12V)
MAX629-02
MAXIMUM LOAD CURRENT vs.
INPUT VOLTAGE (V
OUT
= +24V, +12V)
A: V
OUT
= 12V,
ISET = V
CC
B: V
OUT
= 12V,
ISET = GND
C: V
OUT
=24V,
ISET = V
CC
D: V
OUT
= 24V,
ISET = GND
A
MAX629-03
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
0.1
1
10
D
E, F
D: V
IN
= 5V, ISET = GND
E: V
IN
= 3V, ISET = V
CC
F: V
IN
= 3V, ISET = GND
V
OUT
= 24V
A: V
IN
= 12V, ISET = V
CC
B: V
IN
= 12V, ISET = GND
C: V
IN
= 5V, ISET = V
CC
C
100
95
A
EFFICIENCY (%)
90
B
85
80
75
70
65
60
C
300
250
200
150
100
D
50
0
A
B
MAXIMUM LOAD CURRENT (mA)
C
B
V
OUT
= 12V,
ISET = V
CC
or GND
A: V
IN
= 9V
B: V
IN
= 5V
C: V
IN
= 3V
0.1
1
10
100
100
0
4
8
12
16
20
LOAD CURRENT (mA)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= -18V)
MAX629-04
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= -12V)
MAX629-05
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE
(V
OUT
= -18V, -12V)
80
MAXIMUM LOAD CURRENT (mA)
70
60
50
40
30
20
10
0
100
0
4
8
12
16
20
INPUT VOLTAGE (V)
D
C
B
A: V
OUT
= -12V, ISET = V
CC
B: V
OUT
= -18V, ISET = V
CC
C: V
OUT
= -12V, ISET = GND
D: V
OUT
= -18V, ISET = GND
A
MAX629-06
100
95
90
EFFICIENCY (%)
100
95
90
EFFICIENCY (%)
90
85
80
75
70
65
60
55
50
0.1
1
10
A: V
IN
= 12V, ISET = V
CC
B: V
IN
= 12V, ISET = GND
C: V
IN
= 5V, ISET = V
CC
or GND
D: V
IN
= 3V, ISET = V
CC
or GND
D
B
C
A
85
80
75
70
65
60
55
50
A
B, C
D
A = V
IN
= 5V, ISET = V
CC
B = V
IN
= 5V, ISET = GND
C = V
IN
= 3V, ISET = V
CC
D = V
IN
= 3V, ISET = GND
0.1
1
10
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
_______________________________________________________________________________________
3
28V, Low-Power, High-Voltage,
Boost or Inverting DC-DC Converter
MAX629
____________________________Typical Operating Characteristics (continued)
(SHDN = V
CC
, C
REF
= 0.1µF, T
A
= +25°C, unless otherwise noted.)
SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX629-07
REFERENCE VOLTAGE
vs. REFERENCE LOAD CURRENT
V
IN
= V
CC
= 5V
C4 = 0.47µF
REFERENCE VOLTAGE (V)
1.250
MAX629-08
700
V
IN
= V
CC
600
SUPPLY CURRENT (µA)
500
400
I
IN
300
200
100
0
0
1
2
3
4
5
INPUT VOLTAGE (V)
I
CC
I
IN
1.255
1.245
1.240
1.235
1.230
0
20
40
60
80
100 120 140 160
REFERENCE LOAD CURRENT (µA)
OUTPUT VOLTAGE RIPPLE
MAX629-09
LOAD-TRANSIENT RESPONSE
(I
SET
= V
CC
, I
LIM
= 500mA)
MAX629-10
LOAD-TRANSIENT RESPONSE
(I
SET
= GND, I
LIM
= 250mA)
0mA
A
5mA
MAX629-11
0mA
A
5mA
A
B
B
B
10µs/div
V
OUT
= +24V, I
LOAD
= 5mA
A: ISET = V
CC
, 20mV/div
B: ISET = GND, 20mV/div
200µs/div
V
OUT
= +24V, ISET = V
CC
A: LOAD CURRENT, 0mA TO 5mA, 2.5mA/div
B: V
OUT
, AC-COUPLED, 10mV/div
100µs/div
V
OUT
= +24V, ISET = GND
A: LOAD CURRENT, 0mA TO 5mA, 2.5mA/div
B: V
OUT,
AC-COUPLED, 10mV/div
SHUTDOWN TRANSIENT
(POSITIVE CONFIGURATION)
MAX629-12
SHUTDOWN TRANSIENT
(NEGATIVE CONFIGURATION)
5V
SHDN
0V
MAX629-13
5V
SHDN
0V
24V
V
OUT
0V
0V
V
OUT
-20V
50ms/div
V
CC
= V
IN
= 5V, R
L
= 4kΩ
20ms/div
50ms/div
START-UP
= 5V, R
L
=
CC
= V
IN
= 5V, I
LOAD
= 5mA
V
CC
= V
IN
DELAY, V
4kΩ
4
_______________________________________________________________________________________
28V, Low-Power, High-Voltage,
Boost or Inverting DC-DC Converter
______________________________________________________________Pin Description
PIN
1
2
3
4
5
6
7
8
NAME
SHDN
POL
REF
FB
ISET
GND
LX
V
CC
FUNCTION
Active-Low Shutdown Input. A logic low puts the MAX629 in shutdown mode and reduces supply current to
1µA.
Polarity Input. Changes polarity and threshold of FB to allow regulation of either positive or negative output
voltages. Set POL = GND for positive output voltage, or set POL = V
CC
for negative output voltage.
1.25V Reference Output. Bypass to GND with a 0.1µF capacitor for I
REF
≤
10µA. REF can source 100µA to
drive external loads. For 10µA
≤
I
REF
≤
100µA, bypass REF with a 0.47µF capacitor.
Feedback Input for setting output voltage. Connect to an external voltage divider. See
Setting the Output
Voltage.
Current-Limit Set Input. Connect to V
CC
for a 500mA LX current limit, or connect to GND for a 250mA LX
current limit. See
Setting the Current Limit.
Ground
Internal N-Channel DMOS Switch Drain
Power-Supply Input
MAX629
_______________Detailed Description
The MAX629 low-power, boost DC-DC converter pro-
vides either positive or negative output voltages up to
±28V from a wide range of input voltages. It is
designed primarily for use in low-power, high-voltage
applications such as LCD biasing and set-top box var-
actor tuning. The MAX629’s unique control scheme
provides high efficiency and a wide range of output
voltages with only 80µA quiescent supply current, mak-
ing it ideal for battery-powered applications. The inter-
nal N-channel DMOS switch has a pin-programmable
current limit (250mA and 500mA), allowing optimization
of output current and component size. Figure 1 shows
the MAX629 functional diagram.
ulation, however, the off-time is increased to 5µs to pro-
vide soft-start during start-up. The switching frequency,
which depends upon the load, can be as high as
300kHz.
Shutdown Mode
When
SHDN
is low, the MAX629 enters shutdown
mode. In this mode, the feedback and control circuit,
reference, and internal biasing circuitry turn off. The
shutdown current drops to less than 1µA.
SHDN
is a
logic-level input; connect it to V
CC
for normal operation.
The output voltage behavior in shutdown mode
depends on the output voltage polarity. In the positive
output voltage configuration (Figure 2), the output is
directly connected to the input through the diode (D1)
and the inductor (L1). When the device is in shutdown
mode, the output voltage falls to one diode drop below
the input voltage, and any load connected to the output
may still conduct current. In the negative output voltage
configuration (Figure 3), there is no DC connection
between the input and the output, and in shutdown
mode the output is pulled to GND.
Control Scheme
A combination of peak-current limiting and a pair of
one-shots controls the MAX629 switching, determining
the maximum on-time and constant off-time. During the
on-cycle, the internal switch closes, and current
through the inductor ramps up until either the fixed
10µs maximum on-time expires (at low input voltages)
or the switch’s peak current limit is reached. The peak
switch current limit is selectable to either 500mA (ISET
= V
CC
) or 250mA (ISET = GND) (see
Setting the
Current Limit).
After the on-cycle terminates, the switch
turns off, charging the output capacitor through the
diode. In normal operation, the minimum off-time is set
to 1µs for positive output voltages and 3.5µs for nega-
tive output voltages. When the output is well below reg-
__________________Design Procedure
Setting the Output Voltage
For either positive or negative output voltage applica-
tions, set the MAX629’s output voltage using two exter-
nal resistors, R1 and R2, as shown in Figures 2 and 3.
Since the input bias current at FB has a 50nA maximum
value, large resistors can be used in the feedback loop
5
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