MAX1817EUB [MAXIM]
Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter; 结构紧凑，高效率，双输出升压型DC -DC转换器
| 型号： | MAX1817EUB |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter |
| 文件： | 总12页 (文件大小：403K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1794 Rev 0; 10/00
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
General Description
Features
The MAX1817 is a compact, high-efficiency, dual-out-
put step-up converter for portable devices that pro-
vides both the main logic supply and the LCD bias. The
device operates from an input voltage of +1.5V to
+5.5V, allowing the use of 2- or 3-cell alkaline batteries,
or 1-cell lithium-ion (Li+) batteries.
o Dual Step-Up Converter in a Tiny 10-Pin µMAX
Package
o Main Output
Up to 125mA Load Current
Fixed 3.3V or Adjustable 2.5V to 5.5V
Up to 88% Efficiency
Internal Switch
The MAX1817’s main regulator supplies 125mA at
either a preset 3.3V or an adjustable 2.5V to 5.5V out-
put voltage with up to 88% efficiency. A 0.1µA shut-
down state also minimizes battery drain. The
MAX1817’s secondary step-up converter provides the
LCD bias voltage and is adjustable up to +28V.
o LCD Output
Up to 28V for LCD Bias
Internal Switch
o Input Voltage Range +1.5V to +5.5V
o Minimal External Components Required
o 0.1µA Logic-Controlled Shutdown
o Low 15µA Quiescent Supply Current
Other features include a fast switching frequency to
reduce the size of external components and a low qui-
escent current to maximize battery life. Both outputs can
be independently shut down for improved flexibility.
The MAX1817 is supplied in a compact 10-pin µMAX
package. The MAX1817 evaluation kit (MAX1817EVKIT)
is available to speed up design.
Ordering Information
________________________Applications
PART
TEMP. RANGE
PIN-PACKAGE
Organizers/Translators
MAX1817EUB
-40°C to +85°C
10 µMAX
PDAs
MP3 Players
GPS Receivers
Typical Operating Circuit
Pin Configuration
+1.5V
TO +5.5V
TOP VIEW
FB
ON
1
2
3
4
5
10 OUT
9
8
7
6
LX
LCD
MAX1817
ONLCD
FBLCD
AGND
GND
LXLCD
N.C.
LXLCD
FBLCD
LX
ONLCD
LCD ON/OFF
MAIN ON/OFF
µMAX
MAX1817
ON
AGND
OUT
FB
GND
MAIN
________________________________________________________________ Maxim Integrated Products
1
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
OUT to GND .............................................................-0.3V to +6V
Operating Temperature Range ...........................-40°C to +85°C
ON, ONLCD, FB, FBLCD, LX to GND ......-0.3V to (V
LXLCD to GND .......................................................-0.3V to +30V
AGND to GND .......................................................-0.3V to +0.3V
+ 0.3V)
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
OUT
Continuous Power Dissipation (T = +70°C)
A
10-Pin µMAX (derate 5.6mW/°C above +70°C)...........444mW
LXLCD, LX Maximum Current ........................................0.5A
RMS
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
= V
= V
= +3.3V, FB = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
ON
ONLCD
OUT
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
GENERAL
Input Voltage Range
1.5
5.5
V
V
R
R
= 35Ω
1.5
1.2
LOAD
Startup Voltage
= ∞, V = 1.35V
1.55
10
LOAD
FB
Quiescent Current from OUT
(Main Only)
V
V
= V
= 1.35V,
FB
FBLCD
= 0
5
µA
ONLCD
Quiescent Current from OUT
Shutdown Quiescent Current
MAIN OUTPUT
V
V
= V
= 1.35V
15
30
1
µA
µA
FB
FBLCD
= V
= 0
ONLCD
0.1
ON
V
V
V
rising, V = 1.35V
2.2
2.15
3.3
2.4
OUT
OUT
FB
OUT Undervoltage Lockout
V
V
V
falling, V = 1.35V
1.95
3.14
FB
Fixed-Mode Output Voltage
≤ 45mV
3.47
1.30
FB
Adjustable-Mode FB Regulation
Voltage
1.20
1.25
FB Input Bias Current
V
= 1.35V
50
nA
FB
FB Dual ModeTM Threshold
45
2.5
2.4
0
75
105
mV
Output Voltage Adjustment
Range
5.5
7.5
40
V
Maximum LX On-Time
V
V
= 0.5V
5
µs
FB
Zero Crossing Comparator
20
mV
Threshold (V - V
)
LX
OUT
Zero Crossing Comparator
Backup Timer
= +0.5V
22
45
1.6
1.6
70
µs
%
%
FB
I
= 100mA,
= +2V to +3V
OUT
Line Regulation
Load Regulation
V
IN
V
= +2.5V,
= 10mA to 100mA
IN
I
LOAD
LX On-Resistance
LX Current Limit
V
= 3.3V, I = 100mA
0.35
0.75
0.65
1.05
Ω
OUT
LX
0.5
A
Dual Mode is a trademark of Maxim Integrated Products.
_______________________________________________________________________________________
2
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V
= V
= V = +3.3V, FB = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
OUT A A
ON
ONLCD
PARAMETER
CONDITIONS
= GND, V = 5.5V
MIN
TYP
MAX
1
UNITS
µA
LX Leakage Current
ON Input Low Voltage
ON Input High Voltage
ON Input Bias Current
LCD OUTPUT
V
0.1
ON
LX
1.8V < V
1.8V < V
< 5.5V, V = 0.5V
400
mV
V
OUT
OUT
FB
< 5.5V, V = 0.5V
1.6
FB
1
µA
LXLCD Voltage
28
2.0
0.7
1
V
Ω
LXLCD On-Resistance
LXLCD Current Limit
LXLCD Leakage Current
FBLCD Regulation Voltage
FBLCD Input Bias Current
LCD Line Regulation
LCD Load Regulation
Maximum LXLCD On-Time
V
V
V
= 3.3V, I
= 100mA
1.1
0.5
OUT
LXLCD
0.28
1.20
A
= 28V, V
= 0
ONLCD
0.1
µA
V
LXLCD
FBLCD
1.25
1.30
50
= 1.35V
= 5mA, V = +2V to +3V
nA
%
%
µs
I
I
0.1
0.5
9
LOAD
LOAD
IN
= 1mA to 5mA, V = +2.5V
IN
4
14
1.5
7.5
400
V
V
≥ 1.2V
≤ 0.7V
0.5
2.4
1
FBLCD
FBLCD
Minimum LXLCD Off-Time
µs
5
ONLCD Input Low Voltage
ONLCD Input High Voltage
ONLCD Input Bias Current
2.5V < V
2.5V < V
< 5.5V
mV
V
OUT
OUT
< 5.5V
1.6
1
µA
ELECTRICAL CHARACTERISTICS
(V
= V
= V = +3.3V, FB = GND, T = -40°C to +85°C, unless otherwise noted.) (Note 1)
OUT A
ON
ONLCD
PARAMETER
CONDITIONS
MIN
MAX
UNITS
GENERAL
Input Voltage Range
Startup Voltage
1.5
5.5
1.7
V
V
R
= ∞, V = 1.35V
FB
LOAD
Quiescent Current from OUT
(Main Only)
V
= V
= 1.35V, V = 0
ONLCD
10
µA
FB
FBLCD
Quiescent Current from OUT
Shutdown Quiescent Current
MAIN OUTPUT
V
V
= V
= 1.35V
30
1
µA
µA
FB
FBLCD
= V
= 0
ONLCD
ON
V
V
V
rising, V = 1.35V
2.4
OUT
OUT
FB
OUT Undervoltage Lockout
V
V
falling, V = 1.35V
1.95
3.14
FB
Fixed-Mode Output Voltage
≤ 45mV
3.47
1.30
50
FB
Adjustable-Mode FB Regulation
Voltage
1.20
V
FB Input Bias Current
V
= 1.35V
nA
FB
_______________________________________________________________________________________
3
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V
= V
= V = +3.3V, FB = GND, T = -40°C to +85°C, unless otherwise noted.) (Note 1)
OUT A
ON
ONLCD
PARAMETER
CONDITIONS
MIN
45
MAX
105
5.5
UNITS
mV
V
FB Dual Mode Threshold
Output Voltage Adjustment Range
Maximum LX On-Time
2.5
2.4
V
= 0.5V
= 0.5V
7.5
µs
FB
Zero Crossing Comparator
0
40
70
mV
µs
Threshold (V - V
)
LX
OUT
Zero Crossing Comparator
Backup Timer
V
V
22
FB
LX On-Resistance
= 3.3V, I = 100mA
0.65
1.05
1
Ω
A
OUT
LX
LX Current Limit
0.42
1.6
LX Leakage Current
ON Input Low Voltage
ON Input High Voltage
ON Input Bias Current
LCD OUTPUT
V
= GND, V = 5.5V
µA
mV
V
ON
LX
1.8V < V
1.8V < V
< 5.5V, V = 0.5V
400
OUT
OUT
FB
< 5.5V, V = 0.5V
FB
1
µA
LXLCD Voltage
28
2
V
Ω
LXLCD On-Resistance
LXLCD Current Limit
LXLCD Leakage Current
FBLCD Regulation Voltage
FBLCD Input Bias Current
Maximum LXLCD On-Time
V
V
V
= 3.3V, I
= 100mA
OUT
LXLCD
0.25
1.20
0.7
1
A
= 28V, V
= 0
ONLCD
µA
V
LXLCD
FBLCD
1.30
70
= 1.35V
nA
µs
4
14
V
V
≥ 1.2V
≤ 0.7V
0.5
2.2
1.5
7.5
400
FBLCD
FBLCD
Minimum LXLCD Off-Time
µs
ONLCD Input Low Voltage
ONLCD Input High Voltage
ONLCD Input Bias Current
2.5V < V
2.5V < V
< 5.5V
mV
V
OUT
OUT
< 5.5V
1.6
1
µA
Note 1: Specifications to -40°C are guaranteed by design and not production tested.
4
_______________________________________________________________________________________
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
Typical Operating Characteristics
(Circuit of Figure 3, T = +25°C, unless otherwise noted.)
A
LCD OUTPUT EFFICIENCY
vs. LOAD CURRENT
MAIN OUTPUT EFFICIENCY
vs. LOAD CURRENT
STARTUP VOLTAGE vs. LOAD CURRENT
160
140
120
100
80
75
95
90
85
80
75
70
RESISTIVE LOAD
LCD OFF
A
70
65
60
55
50
45
40
35
B
A: V = 3.3V,
B: V = 2.4V,
IN
OUT
C
IN
OUT
V
= 5V
V
= 5V
D
E
F
A: V = +2.4V, V
IN
= 12V
= 18V
= 24V
= 12V
= 18V
= 24V
IN
LCD
LCD
LCD
LCD
B: V = +2.4V, V
C: V = +2.4V, V
IN
D: V = +1.8V, V
IN
60
D: V = 1.8V,
OUT
IN
= 5V
C: V = 2.4V,
OUT
E: V = +1.8V, V
IN
IN
= 3.3V
IN
LCD
LCD
V
V
F: V = +1.8V, V
40
E: V = 1.8V,
IN
V
= 3.3V
20
OUT
V
= 3.3V,
30
25
OUT
NO LOAD
CIRCUIT OF FIGURE 2
0
0.01
0.1
1
10
100
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
STARTUP VOLTAGE (V)
0.1
1
10
100 1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (LCD OFF)
450
400
350
300
250
200
150
100
50
18
16
14
12
10
8
V
OUT
= 3.3V
V
= 18V, NO LOAD
LCD
R1 = 1MΩ, R2 = 75kΩ
6
4
2
0
0
0
1
2
3
4
5
6
1.0
1.5
2.0
2.5
3.0
3.5
4.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
LCD CONVERTER
SWITCHING WAVEFORM
MAIN CONVERTER
SWITCHING WAVEFORM
MAX1817-07
MAX1817-06
A
A
0
0
B
B
C
C
0
0
4µs/div
10µs/div
A: I
, 500mA/div
A: I , 500mA/div
LX
OUT
C: V , 5V/div
LX
LXLCD
B: V , 100mV/div, AC-COUPLED
LCD
B: V , 50mV/div, AC-COUPLED
C: V
, 10V/div
LXLCD
V
V
= 2.4V, V
LCD
= 3.3V, I
LOAD,LCD
= 0,
V
IN
= 2.4V, V
= 3.3V, I
= 50mA, V
= 0
ONLCD
IN
OUT
= 18V, I
LOAD,OUT
= 5mA
OUT
LOAD,OUT
_______________________________________________________________________________________
5
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 3, T = +25°C, unless otherwise specified)
A
MAIN LOAD TRANSIENT RESPONSE
LCD LOAD TRANSIENT RESPONSE
MAX1817-08
MAX1817-09
A
A
B
B
0
0
400µs/div
200µs/div
A: V , 100mV/div, AC-COUPLED
A: V , 50mV/div, AC-COUPLED
OUT
LCD
B: I
, 50mA/div
B: I
, 10mA/div
LOAD, OUT
= 2.4V, V
LOAD, OUT
= 2.4V, V
V
IN
= 3.3V
OUT
V
IN
= 3.3V (NO LOAD), V
= 18V
LCD
OUT
MAIN LINE TRANSIENT RESPONSE
LCD LINE TRANSIENT RESPONSE
MAX1817-10
MAX1817-11
A
B
A
B
2.4V
1.8V
2.4V
1.8V
400µs/div
100mV/div, AC-COUPLED
200µs/div
A: V , 100mV/div, AC-COUPLED
A: V
IN,
OUT,
B: V 1V/div
LCD
B: V 1V/div
IN,
V
OUT
= 3.3V (NO LOAD), V
= 18V, I
= 2mA
V
OUT
= 3.3V, I
= 20mA, V = 0
ONLCD
LCD
LOAD,LCD
LOAD,MAIN
MAIN OUTPUT TURN-ON/TURN-OFF
RESPONSE
LCD OUTPUT TURN-ON/TURN-OFF
RESPONSE
MAX1817-12
MAX1817-13
A
A
0
0
B
B
0
0
0
0
C
C
100µs/div
400µs/div
A: V , 2V/div
IN
C: V , 5V/div
ON
A: V , 10V/div
LCD
OUT
B: I , 500mA/div
B: I , 200mA/div
IN
C: V
, 5V/div
ONLCD
V
IN
= 2.4V, R
= 165Ω, V
= 0
V
IN
= 2.4V, V
= 3.3V (NO LOAD), R
= 9kΩ
LOAD,LCD
LOAD,MAIN
ONLCD
OUT
6
_______________________________________________________________________________________
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
Pin Description
PIN
NAME
FUNCTION
Main Output Feedback Input. Connect FB to GND for fixed 3.3V main output. For other output
voltages, use a resistive voltage-divider to set the output voltage. The feedback regulation voltage
is 1.25V at FB.
1
FB
Main Step-Up Converter On/Off Control. Connect ON to OUT for automatic startup. Connect ON to
GND to put the IC into shutdown mode.
2
3
4
ON
LCD Output On/Off Control. Connect ONLCD to OUT to enable the LCD output. Connect ONLCD
to GND to disable the LCD output. The main output must be ≥2.4V to enable the LCD output.
ONLCD
FBLCD
LCD Output Feedback Input. Use a resistive voltage-divider from the LCD output to FBLCD to set
the voltage. The feedback regulation voltage is 1.25V at FBLCD.
5
6
AGND
N.C.
Analog Ground. Connect AGND to GND as close to the IC as possible.
No Connection. Not internally connected.
LCD Output Switching Node. Drain of the internal N-channel MOSFET that drives the LCD output.
Connect an external inductor and rectifier to LXLCD.
7
8
9
LXLCD
GND
LX
Power Ground. Connect GND to AGND as close to the IC as possible.
Main Output Switching Node. Drain of the internal N-channel MOSFET that drives the main output.
Connect an external inductor and rectifier to LX.
Main Step-Up Converter Output. OUT is used to measure the output voltage in fixed mode (FB =
GND) and is the internal bias supply input to the IC. When shut down (ON = ONLCD = GND), OUT
is high impedance, drawing 1µA (max).
10
OUT
biasing the internal control circuitry. The MAX1817
________________Detailed Description
switches only as often as is required to supply sufficient
power to the load. This allows the converter to operate
at lower frequencies at light loads, improving efficiency.
The MAX1817 dual step-up converter is designed to
supply the main power and LCD bias for low-power,
hand-held devices. The MAX1817’s main step-up con-
verter includes a 0.35Ω N-channel power MOSFET
switch and provides a fixed 3.3V or adjustable 2.5V to
5.5V output at up to 125mA from an input as low as
1.5V. The MAX1817’s LCD bias step-up converter
includes a high-voltage 1.1Ω power MOSFET switch to
support as much as 5mA at 28V (Figure 1). During
startup, the MAX1817 extends the LCD MOSFET switch
minimum off-time, limiting surge current. Both convert-
ers require an inductor and external rectifier.
The control scheme maintains regulation when the error
amplifier senses the output voltage is below the feed-
back threshold, turning on the internal N-channel MOS-
FET and initiating an on-time. The on-time is terminated
when the 0.75A current limit is reached or when the
maximum on-time is reached. The N-channel MOSFET
remains off until the inductor current drops to 0, forcing
discontinuous inductor current. At the end of a cycle,
the error comparator waits for the voltage at FB to drop
below the regulation threshold, at which time another
cycle is initiated.
The MAX1817 runs in bootstrap mode, powering the IC
from the main step-up converter’s output. Independent
logic-controlled shutdown for the main and LCD step-
up converters reduces quiescent current to 0.1µA.
The main step-up converter uses a startup oscillator to
allow it to start from an input voltage as low as +1.2V.
This is necessary since the control circuitry is powered
from the step-up converter output (OUT). When the
voltage at OUT is below the OUT undervoltage lockout,
a fixed 50% duty cycle drives the internal N-channel
MOSFET, forcing the main output voltage to rise. Once
Main Step-Up Converter
The MAX1817 main step-up converter runs from a
+1.5V to +5.5V input voltage and produces a fixed 3.3V
or adjustable 2.5V to 5.5V output voltage as well as
_______________________________________________________________________________________
7
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
V
IN
DUAL-MODE FEEDBACK
OUT
LX
ZERO-
CROSSING
DETECTOR
MAIN
ERROR
COMPARATOR
MAIN
CONTROL
LOGIC
MAIN
FB
1.25V
AGND
75mV
STARTUP
CURRENT
LIMIT
MAIN
SHUTDOWN
LOGIC
ON
ON
ON
GND
OFF
OFF
MAIN
LCD
LXLCD
UNDERVOLTAGE
LOCKOUT
SHUTDOWN
LOGIC
LCD
CONTROL
LOGIC
LCD
ONLCD
BIAS
LCD
ERROR
COMPARATOR
LCD
1.25V
AGND
CURRENT
LIMIT
LCD
MAX1817
GND
FBLCD
AGND
GND
Figure 1. MAX1817 Simplified Functional Diagram
the output voltage rises above the undervoltage thresh-
old, the control circuitry is enabled, allowing proper
regulation of the output voltage.
During startup, the MAX1817 extends the minimum off-
time to 5µs for V voltages <0.9V, limiting initial
surge current. The LCD step-up converter features an
independent shutdown control, ONLCD.
FBLCD
LCD Step-Up Converter
The MAX1817’s LCD step-up converter generates an
LCD bias voltage up to 28V by use of a 500mA, 1.1Ω
internal N-channel switching MOSFET (Figure 1). The
LCD step-up converter control circuitry is powered from
the main step-up converter output (OUT), so the voltage
at OUT must be above the OUT undervoltage lockout
voltage for the LCD step-up converter to operate.
The LCD step-up converter features a minimum-off-
time, current-limited control scheme. A pair of one-
shots that set a minimum off-time and a maximum on-
time governs the duty cycle. The switching frequency
can be up to 500kHz and depends upon the load, and
input and output voltages.
8
_______________________________________________________________________________________
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
C1
10µF
C1
10µF
V
IN
V
IN
L1
10µH
L1
10µH
L2
10µH
L2
10µH
D1
D1
D2
D2
18V
LCD
C2
1µF
LCD 18V
C2
1µF
LXLCD
LX
4.7pF
C4
R1
1M
LXLCD
LX
4.7pF
C4
R1
1M
ON
LCD
ON
LCD
OFF
OFF
OFF
OFF
ONLCD
FBLCD
OUT
ONLCD
FBLCD
OUT
ON
R2 75k
MAIN 5V
ON
R2 75k
MAIN 3.3V
MAX1817
ON
MAIN
MAX1817
ON
C3
22µF
MAIN
R3
C3
22µF
300k
FB
FB
AGND
GND
R4
100k
AGND
GND
Figure 2. Setting Main Output Voltage Using External Resistors
Figure 3. Typical Application Circuit
Low-Voltage Startup
The MAX1817’s internal circuitry is powered from OUT.
The main step-up converter has a low-voltage startup
circuit to control main DC-DC converter operation until
___________________Design Procedure
Setting the Main Output Voltage
The main step-up converter feedback input (FB) fea-
tures Dual Mode operation. With FB grounded, the
main output voltage is preset to 3.3V. It can also be
adjusted from 2.5V to 5.5V with external resistors R3
and R4 as shown in Figure 2. To set the output voltage
externally, select resistor R4 from 10kΩ to 100kΩ.
Calculate R3 using:
V
exceeds the 2.2V (typ) undervoltage lockout
OUT
threshold. The minimum startup voltage is a function of
load current (see Typical Operating Characteristics).
The MAX1817 main converter typically starts up into a
35Ω load with input voltages down to +1.5V, allowing
startup with two alkaline cells even in deep discharge.
R3 = R4 [(V
/ V ) – 1]
OUT
FB
Shutdown: ON and ONLCD
The MAX1817 features independent shutdown control
of the main and LCD step-up converters. With both
converters shut down, supply current is reduced to
0.1µA. A logic low at ON shuts down the main step-up
converter, and LX enters a high-impedance state.
However, the main output remains connected to the
input through the inductor and output rectifier, holding
where V = 1.25V, and V
FB
can range from 2.5V to
OUT
5.5V.
Setting the LCD Output Voltage
Set the LCD output voltage with two external resistors
R1 and R2 as shown in Figure 3. Since the input leak-
age current at FBLCD has a maximum of 50nA, large
resistors can be used without significant accuracy loss.
Begin by selecting R2 in the 10kΩ to 100kΩ range, and
calculate R1 using the following equation:
V
to one diode drop below the input voltage when
OUT
the main converter is shut down. If the input voltage is
sufficiently high to drive V
above the undervoltage
OUT
R1 = R2 [(V
/ V
) – 1]
LCD
FBLCD
lockout voltage, the LCD step-up converter operates.
where V
to 28V.
= 1.25V, and V
can range from V
FBLCD
LCD
IN
A logic low at ONLCD shuts down the LCD step-up
converter, and LXLCD enters a high-impedance state.
The LCD output remains connected to the input
through the inductor and output rectifier, holding it to
one diode drop below the input.
_______________________________________________________________________________________
9
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
shown in Figure 3 are recommended for most applica-
C6
0.1µF
R3
1
tions, although values between 4.7µH and 47µH are
suitable. Smaller inductance values typically offer a
smaller physical size for a given series resistance,
allowing the smallest overall circuit dimensions. Larger
inductance values exhibit higher output current capa-
bility, but larger physical dimensions.
-19V
LCD
D3*
L1,10µH
V
C5
1µF
C1
10µF
V
IN
D4*
L2
10µH
Circuits using larger inductance values may start up at
lower input voltages and exhibit less ripple, but they
may provide reduced output power. This occurs when
the inductance is sufficiently large to prevent the maxi-
mum current limit from being reached before the maxi-
mum on-time expires. The inductor’s saturation current
rating should be greater than the peak switching cur-
rent. However, it is generally acceptable to bias most
inductors into saturation by as much as 20%, although
this may slightly reduce efficiency.
D2
LX
D1**
R1
LXLCD
C4
C2
0.1µF
240k
10pF
MAX1817
ONLCD
FBLCD
R2
16.5k
ON
OUT
FB
MAIN
C3
22µF
For best efficiency, select inductors with resistance no
greater than the internal N-channel FET resistance in
each step-up converter.
AGND
GND
*D3, D4 = CENTRAL SEMICONDUCTOR
CMPD7000 DUAL
**D1 = CENTRAL SEMICONDUCTOR
CMSD4448 (1N4148)
For maximum output current, choose L such that:
✕
L < [(V
t
) / I
]
IN
ON
PEAK
where t
is the maximum switch on-time (5µs for main
Figure 4. Negative Voltage for LCD Bias
ON
step-up converter) or 9µs for LCD step-up converter)
and I is the switch peak current limit (0.75A for the
main step-up converter, or 0.5A for the LCD step-up
converter). With this inductor value, the maximum output
current the main converter is able to deliver is given by:
PEAK
Using a Charge Pump to Make Negative
LCD Output Voltage
The MAX1817 can generate a negative LCD output by
adding a diode-capacitor charge-pump circuit (D3, D4,
and C6) to the LXLCD pin as shown in Figure 4. FBLCD
is driven through a resistive voltage-divider from the
positive output, which is not loaded, allowing a very
small capacitor value at C2. For best stability and low-
est ripple, the time constant of the R1 + R2 series com-
bination and C2 should be near that of C5 and the
effective load resistance. Output load regulation of the
negative output degrades compared to the standard
positive output circuit and may rise at very light loads. If
this is not acceptable, reduce the resistance of R1 and
R2, while maintaining their ratio, to effectively preload
the output with a few hundred µA. This is why the R1
and R2 values shown in Figure 4 are lower than typical
values for a positive-output design. When loaded, the
magnitude of the negative output voltage is slightly
lower (closer to ground by approximately a diode for-
ward voltage) than the voltage on C2.
✕
I
= 0.5
I
/ (1 + t
/ t
)
OUT(MAX)
PEAK
ON OFF
where t
/ t
= (V
+ V - V ) / (V - V ), V
ON OFF
OUT D IN IN ON IN
and V
are the input and output voltages, V is the
OUT
D
✕
Schottky diode drop (0.3V typ), and V
= I
ON
PEAK
R , where R
ON
is the switch on-resistance.
ON
For V = 1.5V and V
IN
= 3.3V, with a minimum I
OUT
PEAK
(0.65) =
✕
value of 0.5A, and V
given by (0.5)
ON(MAX)
0.325V, the available output current that the converter
can provide is at least 90mA.
For larger inductor values, I
is determined by:
PEAK
✕
I
= [(V
t ) / L]
ON
PEAK
IN
External Rectifiers
The high maximum switching frequency of the
MAX1817 requires a high-speed rectifier. Schottky
diodes such as the Motorola MBR0530 or the Nihon
EP05Q03L are recommended. To maintain high effi-
ciency, the average current rating of the Schottky diode
should be greater than the peak switching current. A
junction diode such as the Central Semiconductor
CMPD4448 can be used for the LCD output with little
Applications Information
Inductor Selection
The MAX1817’s high switching frequency allows the
use of small surface-mount inductors. The 10µH values
10 ______________________________________________________________________________________
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
loss in efficiency. Choose a reverse breakdown voltage
greater than the output voltage.
(main converter) from 2.5V to 5.5V may require a larger
value LCD feed-forward capacitor to prevent multipuls-
ing of the LCD converter. Larger feed-forward capaci-
tors slightly degrade load regulation, so choose the
smallest value capacitor that provides stability.
Input Bypass Capacitor
The input supplies high currents to the inductors and
requires local bulk bypassing close to the inductors. A
low equivalent series resistance (ESR) input capacitor
connected in parallel with the battery will reduce peak
battery currents and input-reflected noise. Battery
bypassing is especially helpful at low input voltages
and with high-impedance batteries (such as alkaline
types). Benefits include improved efficiency and lower
useful end-of-life voltage for the battery. A single 10µF
low-ESR surface-mount capacitor is sufficient for most
applications.
Layout Considerations
The MAX1817’s high-frequency operation makes PC
board layout important for optimal performance. Use
separate analog and power ground planes. Connect
the two planes together at a single point as close as
possible to the IC. Use surface-mount components
where possible. If leaded components are used, mini-
mize lead lengths to reduce stray capacitance and
keep the components close to the IC to minimize trace
resistance. Where an external voltage-divider is used to
set output voltage, the traces from FB or FBLCD to the
feedback resistors should be extremely short (less than
0.2in or 5mm) to minimize coupling from LX and
LXLCD. Refer to the MAX1817 evaluation kit for a full
PC board example.
Output Bypass Capacitors
For most applications, use a small surface-mount 22µF
or greater ceramic capacitor on the main converter out-
put, and a 1µF or greater ceramic capacitor on the LCD
output. For small ceramic capacitors, the output ripple
voltage is dominated by the capacitance value. If tanta-
lum or electrolytic capacitors are used, the ESR of the
capacitors dominates the output ripple voltage.
Decreasing the ESR reduces the output ripple voltage
and the peak-to-peak transient voltage.
____________________Chip Information
TRANSISTOR COUNT: 2785
PROCESS: BiCMOS
LCD Compensation
The MAX1817’s LCD step-up converter feedback
requires a small 4.7pF feed-forward capacitor for the
typical application circuit. Circuits with adjustable V
OUT
______________________________________________________________________________________ 11
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
Package Information
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2000 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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