General Description
The MAX1790/MAX8715 boost converters incorporate
high-performance (at 1.2MHz), current-mode, fixed-frequency, pulse-width modulation (PWM) circuitry with a
built-in 0.21Ω/0.15Ω n-channel MOSFET to provide a highly efficient regulator with fast response.
High switching frequency (640kHz or 1.2MHz selectable)
allows easy filtering and faster loop performance. An
external compensation pin provides the user flexibility in
determining loop dynamics, allowing the use of small, low
equivalent-series-resistance (ESR) ceramic output capacitors. The device can produce an output voltage as high as
12V from an input as low as 2.6V.
Soft-start is programmed with an external capacitor, which
sets the input-current ramp rate. In shutdown mode, current consumption is reduced to 0.1µA. The MAX1790/
MAX8715 are available in a space-saving 8-pin µMAX
®
package. The ultra-small package and high switching frequency allow the total solution to be less than 1.1mm high.
Applications
LCD Displays
PCMCIA Cards
Portable Applications
Hand-Held Devices
Features
♦ 90% Efficiency
♦ Adjustable Output from V
IN
to 12V
♦ 1.6A, 0.21Ω, 14V Power MOSFET (MAX1790)
♦ 2.4A, 0.15Ω, 14V Power MOSFET (MAX8715)
♦ +2.6V to +5.5V Input Range
♦ Pin-Selectable 640kHz or 1.2MHz Switching
Frequency
♦ 0.1µA Shutdown Current
♦ Programmable Soft-Start
♦ Small 8-Pin µMAX Package
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
________________________________________________________________ Maxim Integrated Products 1
IN
LXGND
1
2
87SS
FREQFB
COMP
µMAX
TOP VIEW
3
4
6
5
MAX1790
MAX8715
SHDN
Typical Operating Circuit
19-1563; Rev 2; 5/04
Pin Configuration
Ordering Information
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART TEMP RANGE PIN-PACKAGE
MAX1790EUA -40°C to +85°C 8 µMAX
MAX8715EUA -40°C to +85°C 8 µMAX
µMAX is a registered trademark of Maxim Integrated Products,
Inc.
V
IN
2.6V TO 5V
IN
ON/OFF
SHDN
FREQ
SS
MAX1790
MAX8715
COMP
LX
GND
FB
V
OUT
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= SHDN = 3V, FREQ = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°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.
LX to GND ..............................................................-0.3V to +14V
IN, SHDN, FREQ, FB to GND................................-0.3V to +6.2V
SS, COMP to GND.......................................-0.3V to (V
IN
+ 0.3V)
RMS LX Pin Current ..............................................................1.2A
Continuous Power Dissipation (T
A
= +70°C)
8-Pin µMAX (derate 4.1mW/°C above +70°C).............330mW
Operating Temperature Range
MAX1790EUA/MAX8715EUA ........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETER
CONDITIONS
UNITS
Input Supply Range V
IN
2.6 5.5 V
VIN Undervoltage Lockout UVLO
V
IN
rising, typical hysteresis is 40mV,
LX remains off below this level
V
MAX1790
V
FB
= 1.0V, switching 2 5
Quiescent Current I
IN
MAX8715
V
FB
= 1.0V, switching 2.5 5.0
mA
Shutdown Supply Current I
IN
SHDN = GND 0.1 10 µA
ERROR AMPLIFIER
Feedback Voltage V
FB
Level to produce V
COMP
= 1.24V
V
MAX1790 0 40
FB Input Bias Current I
FB
VFB = 1.24V
MAX8715
nA
Feedback-Voltage Line
Regulation
Level to produce V
COMP
= 1.24V,
2.6V < V
IN
< 5.5V
%/V
MAX1790 70
Transconductance g
m
∆I = 5µA
MAX8715 70
µS
Voltage Gain A
V
V/V
OSCILLATOR
FREQ = GND
Frequency f
OSC
FREQ = IN
kHz
FREQ = GND
79 85 92
Maximum Duty Cycle DC
FREQ = IN 84
%
N-CHANNEL SWITCH
MAX1790 1.2 1.6 2.3
Current Limit I
LIM
VFB = 1V,
duty cycle =
65% (Note 1)
MAX8715 1.8 2.4 3.4
A
MAX1790
0.5
On-Resistance R
ON
MAX8715
Ω
MAX1790
20
Leakage Current I
LXOFF
VLX = 12V
MAX8715 5 30
µA
SYMBOL
MIN TYP MAX
2.25 2.38 2.52
VFB = 1.3V, not switching 0.18 0.35
VFB = 1.3V, not switching 0.21 0.35
1.222 1.24 1.258
540 640 740
1000 1220 1500
125 190
0.05 0.15
140 240
160 240
700
0.21
0.15 0.35
0.01
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS
(VIN= SHDN = 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
ELECTRICAL CHARACTERISTICS (continued)
(VIN= SHDN = 3V, FREQ = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER
CONDITIONS
UNITS
MAX1790
Current-Sense Transresistance R
CS
MAX8715
V/A
SOFT-START
Reset Switch Resistance
Ω
Charge Current VSS = 1.2V 1.5 4 7.0 µA
CONTROL INPUTS
Input Low Voltage V
IL
SHDN, FREQ
V
Input High Voltage V
IH
SHDN, FREQ
V
Hysteresis SHDN, FREQ
V
FREQ Pulldown Current I
FREQ
1.8 5 9.0 µA
SHDN Input Current I
SHDN
1µA
SYMBOL
MIN TYP MAX
0.30 0.45 0.65
0.20 0.30 0.43
0.3 x V
0.7 x V
IN
0.1 x V
IN
0.001
Input Supply Range V
VIN Undervoltage Lockout UVLO
Quiescent Current I
Shutdown Supply Current I
ERROR AMPLIFIER
Feedback Voltage V
FB Input Bias Current I
Feedback-Voltage Line
Regulation
Transconductance g
OSCILLATOR
Frequency f
Maximum Duty Cycle DC FREQ = GND 78 92 %
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
IN
V
rising, typical hysteresis is 40mV,
IN
LX remains off below this level
MAX1790
IN
MAX8715
IN
FB
FB
OSC
SHDN = GND 10 µA
Level to produce V
VFB = 1.24V
Level to produce V
2.6V < V
∆I = 5µA
m
FREQ = GND
FREQ = IN 900 1500
< 5.5V
IN
VFB = 1.3V, not switching 0.35
V
= 1.0V, switching 5
FB
VFB = 1.3V, not switching 0.35
V
= 1.0V, switching 5
FB
= 1.24V 1.215 1.24 1.260 V
COMP
MAX1790 40
MAX8715 190
= 1.24V,
COMP
MAX1790 70 260
MAX8715 70 260
2.6 5.5 V
2.25 2.52 V
490 770
100
IN
mA
nA
0.15 %/V
µS
kHz
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VIN= SHDN = 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
UNITS
N-CHANNEL SWITCH
MAX1790 1.2 2.3
Current Limit I
LIM
VFB = 1V,
duty cycle =
65% (Note 1)
MAX8715 1.8 3.0
A
MAX1790 0.5
On-Resistance R
ON
MAX8715
Ω
MAX1790
Current-Sense Transresistance R
CS
MAX8715
V/A
CONTROL INPUTS
Input Low Voltage V
IL
SHDN, FREQ
V
Input High Voltage V
IH
SHDN, FREQ
V
Note 1: Current limit varies with duty cycle due to slope compensation. See the Output-Current Capability section.
Note 2: Specifications to -40°C are guaranteed by design and not production tested.
SYMBOL
MIN TYP MAX
0.30 0.65
0.20 0.43
0.7 x V
0.35
0.3 x V
IN
IN
Typical Operating Characteristics
(Circuit of Figure 1, VIN= 3.3V, f
OSC
= 640kHz, TA= +25°C, unless otherwise noted.)
MAX1790 toc01
50
1100010010
MAX1790 EFFICIENCY
vs. OUTPUT CURRENT
65
55
85
75
95
70
60
90
80
OUTPUT CURRENT (mA)
EFFICIENCY (%)
f
OSC
= 1.2MHz
L
= 2.7µH
V
IN
= 3.3V
V
OUT
= 5V
f
OSC
= 640kHz
L
= 5.4µH
50
1 100010010
MAX1790 EFFICIENCY
vs. OUTPUT CURRENT
65
55
85
75
95
70
60
90
80
MAX1790 toc02
OUTPUT CURRENT (mA)
EFFICIENCY (%)
f
OSC
= 1.2MHz
L
= 5.4µH
V
IN
= 3.3V
V
OUT
= 12V
f
OSC
= 640kHz
L
= 10µH
OUTPUT CURRENT (mA)
50
1 100010010
MAX1790 EFFICIENCY
vs. OUTPUT CURRENT
65
55
85
75
95
70
60
90
80
MAX1790 toc03
EFFICIENCY (%)
f
OSC
= 1.2MHz
L
= 5.4µH
V
IN
= 5V
V
OUT
= 12V
f
OSC
= 640kHz
L
= 10µH
0
0.2
0.1
0.4
0.3
0.6
0.5
0.7
2.5 3.5 4.03.0 4.5 5.0 5.5
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1790 toc04
INPUT VOLTAGE (V)
NO-LOAD SUPPLY CURRENT (mA)
V
OUT
= 12V
f
OSC
= 1.2MHz
f
OSC
= 640kHz
11.60
11.70
11.65
11.80
11.75
11.90
11.85
11.95
12.05
12.00
12.10
040608020 100 120 140 180160 200
MAX1790 OUTPUT VOLTAGE
vs. OUTPUT CURRENT
MAX1790 toc05
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
f
OSC
= 640kHz
TA = +85°C
TA = +25°C
TA = -40°C
MAX8715 EFFICIENCY
vs. OUTPUT CURRENT
MAX1790 toc06
OUTPUT CURRENT (mA)
EFFICIENCY (%)
10010
50
55
60
65
70
75
80
85
90
95
45
1 1000
VIN = 5.0V
VIN = 3.3V
V
OUT
= 9V
f
OSC
= 1.2MHz
L = 6.8µH
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
_______________________________________________________________________________________ 5
500mA
20mA
CH1 = LOAD CURRENT, 500mA/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
V
OUT
= 5V, f
OSC
= 640kHz, C
OUT
= 47µF + 0.1µF
100µs/div
MAX1790 LOAD-TRANSIENT RESPONSE
MAX1790 toc10
CH1
CH2
CH3
R
COMP
= 62k
Ω
C
COMP
= 820pF
C
COMP2
= 56pF
100µs/div
MAX1790 STARTUP WAVEFORM
WITHOUT SOFT-START
MAX1790 toc11
CH1
CH2
CH3
CH1 = SHDN, 5V/div
CH2 = OUTPUT VOLTAGE, 5V/div
CH3 = INDUCTOR CURRENT, 1A/div
V
IN
= 3.3V, V
OUT
= 12V, I
OUT
= 10mA, f
OSC
= 640kHz
NO SOFT-START CAPACITOR, C
OUT
= 33µF
1ms/div
STARTUP WAVEFORM
WITH SOFT-START
MAX1790 toc12
CH1
CH2
CH3
CH1 = SHDN, 5V/div
CH2 = OUTPUT VOLTAGE, 5V/div
CH3 = INDUCTOR CURRENT, 200mA/div
V
OUT
= 12V, I
OUT
= 10mA, f
OSC
= 640kHz,
C
SS
= 0.027µF, C
OUT
= 33µF
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= 3.3V, f
OSC
= 640kHz, TA= +25°C, unless otherwise noted.)
200mA
10mA
CH1
0
CH2
CH3
CH1 = LOAD CURRENT, 200mA/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 100mV/div
CH3 = INDUCTOR CURRENT, 500mA/div
V
IN
= 3.3V, V
OUT
= 9.0V
f
OSC
= 1.2MHz, L = 6.8µH, C
OUT
= 3 x 3.3µF
40µs/div
MAX8715 LOAD-TRANSIENT RESPONSE
MAX1790 toc07
R
COMP
= 82k
Ω
C
COMP
= 750pF
C
COMP2
= 10pF
1A
40mA
CH1
CH2
CH3
CH1 = LOAD CURRENT, 1A/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 100mV/div
CH3 = INDUCTOR CURRENT, 500mA/div
V
IN
= 3.3V, V
OUT
= 9.0V
f
OSC
= 1.2MHz, L = 6.8µH, C
OUT
= 3 x 3.3µF
10µs/div
MAX8715
PULSED LOAD-TRANSIENT RESPONSE
MAX1790 toc08
200mA
10mA
CH1
CH2
CH3
CH1 = LOAD CURRENT, 100mA/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
V
IN
= 3V
V
OUT
= 12V, f
OSC
= 640kHz, C
OUT
= 33µF + 0.1µF
100µs/div
MAX1790 LOAD-TRANSIENT RESPONSE
MAX1790 toc09
R
COMP
= 120k
Ω
C
COMP
= 1200pF
C
COMP2
= 56pF
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
6 _______________________________________________________________________________________
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
_______________________________________________________________________________________ 7
2ms/div
STARTUP WAVEFORM
WITH SOFT-START
MAX1790 toc13
CH1
CH2
CH3
CH1 = SHDN, 5V/div
CH2 = V
OUT,
5V/div
CH3 = INDUCTOR CURRENT, 500mA/div
V
OUT
= 12V, I
OUT
= 200mA, f
OSC
= 640kHz,
C
SS
= 0.027µF
CH1
CH2
CH3
CH1 = LX SWITCHING WAVEFORM, 5V/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
V
OUT
= 12V, I
OUT
= 200mA, f
OSC
= 640kHz, L = 10µH;
C
OUT
= 33µF + 0.1µF
500ns/div
SWITCHING WAVEFORM
MAX1790 toc14
0
400
200
800
600
1000
1200
1600
1400
1800
3.0 3.4 3.6 3.83.2 4.0 4.2 4.4 4.84.6 5.0
MAX1790 MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
MAX1790 toc15
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
f
OSC
= 640kHz
V
OUT
= 12V
V
OUT
= 5V
0
400
200
800
600
1000
1200
1600
1400
1800
3.0 3.4 3.6 3.83.2 4.0 4.2 4.4 4.84.6 5.0
MAX8715 MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
MAX1790 toc16
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
V
OUT
= 9V
f
OSC
= 1.2MHz
L = 6.8µH
C
OUT
= 3 x 3.3µF
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= 3.3V, f
OSC
= 640kHz, TA= +25°C, unless otherwise noted.)
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
8 _______________________________________________________________________________________
Detailed Description
The MAX1790/MAX8715 are highly efficient power supplies that employ a current-mode, fixed-frequency
PWM architecture for fast transient response and lownoise operation. The device regulates the output voltage through a combination of an error amplifier, two
comparators, and several signal generators (Figure 2).
The error amplifier compares the signal at FB to 1.24V
and varies the COMP output. The voltage at COMP
determines the current trip point each time the internal
MOSFET turns on. As the load varies, the error amplifier
sources or sinks current to the COMP output accordingly to produce the inductor peak current necessary to service the load. To maintain stability at high duty cycle, a
slope-compensation signal is summed with the currentsense signal.
At light loads, this architecture allows the ICs to “skip”
cycles to prevent overcharging the output voltage. In
this region of operation, the inductor ramps up to a
fixed peak value (approximately 50mA, MAX1790 or
75mA, MAX8715), discharges to the output, and waits
until another pulse is needed again.
Pin Description
Switch Pin. Connect the inductor/catch diode to LX and minimize the trace area for lowest EMI.LX5
Supply Pin. Bypass IN with at least a 1µF ceramic capacitor directly to GND.IN6
Frequency Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high,
the frequency is 1.2MHz. This input has a 5µA pulldown current.
FREQ7
Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The softstart capacitor is charged with a constant current of 4µA. Full current limit is reached after t = 2.5
x 10
5
CSS.
The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start
capacitor is charged to 0.5V, after which soft-start begins.
SS8
GroundGND4
Shutdown Control Input. Drive SHDN low to turn off the MAX1790/MAX8715. SHDN
3
PIN
Feedback Pin. Reference voltage is 1.24V nominal. Connect an external resistor-divider tap to FB and
minimize the trace area. Set V
OUT
according to: V
OUT
= 1.24V (1 + R1 / R2). See Figure 1.
FB2
Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop
Compensation section for component selection guidelines.
COMP1
FUNCTIONNAME
Figure 1. Typical Application Circuit
V
IN
2.6V TO 5.5V
IN
1.2MHz
640kHz
0.027µF
ON/OFF
V
SHDN
IN
FREQ
SS
C
COMP2
MAX1790
MAX8715
COMP
R
C
LX
GND
FB
COMP
COMP
L
MBRS130LT1
0.1µF*
C
IN
C1
10µF
6.3V
V
OUT
D1
C
OUT
R1
R2
* OPTIONAL
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
_______________________________________________________________________________________ 9
Output-Current Capability
The output-current capability of the MAX1790/MAX8715
is a function of current limit, input voltage, operating frequency, and inductor value. Because of the slope compensation used to stabilize the feedback loop, the duty
cycle affects the current limit. The output-current capability is governed by the following equation:
I
OUT(MAX)
= [I
LIM
x (1.26 - 0.4 x Duty) -
0.5 x Duty x V
IN
/ (f
OSC
x L)] x η x VIN/ V
OUT
where:
I
LIM
= current limit specified at 65% (see the Electrical
Characteristics)
Duty = duty cycle = (V
OUT
- VIN+ V
DIODE
) /
(V
OUT
- I
LIM
x RON+ V
DIODE
)
V
DIODE
= catch diode forward voltage at I
LIM
η = conversion efficiency, 85% nominal
Soft-Start
The MAX1790/MAX8715 can be programmed for softstart upon power-up with an external capacitor. When the
shutdown pin is taken high, the soft-start capacitor (C
SS
)
is immediately charged to 0.5V. Then the capacitor is
charged at a constant current of 4µA (typ). During this
time, the SS voltage directly controls the peak inductor
current, allowing 0A at V
SS
= 0.5V to the full current limit
at VSS= 1.5V. The maximum load current is available
after the soft-start cycle is completed. When the shutdown pin is taken low, the soft-start capacitor is
discharged to ground.
Frequency Selection
The MAX1790/MAX8715s’ frequency can be user
selected to operate at either 640kHz or 1.2MHz.
Connect FREQ to GND for 640kHz operation. For a
1.2MHz switching frequency, connect FREQ to IN. This
allows the use of small, minimum-height external components while maintaining low output noise. FREQ has
an internal pulldown, allowing the user the option of
leaving FREQ unconnected for 640kHz operation.
Shutdown
The MAX1790/MAX8715 are shut down to reduce the
supply current to 0.1µA when SHDN is low. In this
mode, the internal reference, error amplifier, comparators, and biasing circuitry turn off while the n-channel
MOSFET is turned off. The boost converter’s output is
connected to IN by the external inductor and catch
diode.
Applications Information
Boost DC-DC converters using the MAX1790/MAX8715
can be designed by performing simple calculations for
a first iteration. All designs should be prototyped and
tested prior to production. Table 1 provides a list of
GND
LX
IN
FREQ
FB
COMP
4µA
5µA
N
ERROR
COMPARATOR
ERROR
AMPLIFIER
SKIP
COMPARATOR
SS
CLOCK
SKIP
BIAS
SHDN
MAX1790
MAX8715
Σ
CURRENT
SENSE
CONTROL
AND DRIVER
LOGIC
SOFTSTART
SLOPE
COMPEN-
SATION
OSCILLATOR
∞
1.24V
Figure 2. Functional Diagram
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
10 ______________________________________________________________________________________
components for a range of standard applications. Table
2 lists component suppliers.
External component value choice is primarily dictated
by the output voltage and the maximum load current,
as well as maximum and minimum input voltages.
Begin by selecting an inductor value. Once L is known,
choose the diode and capacitors.
Inductor Selection
The minimum inductance value, peak current rating, and
series resistance are factors to consider when selecting
the inductor. These factors influence the converter’s efficiency, maximum output load capability, transientresponse time, and output voltage ripple. Physical size
and cost are also important factors to be considered.
The maximum output current, input voltage, output voltage, and switching frequency determine the inductor
value. Very high inductance values minimize the current
ripple and therefore reduce the peak current, which
decreases core losses in the inductor and I2R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of wire,
which increase physical size and can increase I2R losses in the inductor. Low inductance values decrease the
physical size but increase the current ripple and peak
current. Finding the best inductor involves choosing the
best compromise between circuit efficiency, inductor
size, and cost.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full load current.
The best trade-off between inductor size and circuit
efficiency for step-up regulators generally has an LIR
between 0.3 and 0.5. However, depending on the AC
characteristics of the inductor core material and the
Table 1. Component Selection
Table 2. Component Suppliers
847-639-6400
561-241-7876
847-956-0666
PHONE
847-639-1469Coilcraft
561-241-9339Coiltronics
847-956-0702Sumida USA
FAXSUPPLIER
803-946-0690
408-986-0424
619-661-6835
847-297-0070
803-626-3123AVX
408-986-1442Kemet
619-661-1055Sanyo
847-699-1194TOKO
516-435-1110
310-322-3331
516-543-7100
602-303-5454
408-573-4150
847-843-7500
516-864-7630Zetex
847-843-2798Nihon
516-435-1824
Central
Semiconductor
310-322-3332
International
Rectifier
602-994-6430Motorola
408-573-4159Taiyo Yuden
Inductors
Capacitors
Diodes
V
VIN (V)
MAX1790
3.3 12 640k
3.3 12 1.2M
3.3 5 640k
3.3 5 1.2M
MAX8715
3.3 9 1.2M
OUT
(V)
f
OSC
(Hz)
L (µH) C
10 (Sumida
CDRH5D18-100NC)
5.4 (Sumida
CDRH5D18-5R4NC)
5.4 (Sumida
CDRH5D18-5R4NC)
2.7 (Sumida
CDRH4D18-2R7)
6.8 (Sumida
CLQ4D10-6R8)
(µF)
OUT
33 tantalum (AVX
TPSD336020R0200)
33 tantalum (AVX
TPSD336020R0200)
47 tantalum
(6TPA47M)
47 tantalum
(6TPA47M)
3 x 3.3 ceramic
(Taiyo Yuden
LMK325BJ335MD)
R
COMP
(kΩ)
120 1200 22 250
180 650 20 250
62 820 56 800
91 390 33 800
82 750 10 150
C
COMP
(pF)
C
COMP2
(pF)
I
OUT(MAX)
(mA)
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
______________________________________________________________________________________ 11
ratio of inductor resistance to other power path resistances, the best LIR can shift up or down. If the inductor resistance is relatively high, more ripple can be
accepted to reduce the number of turns required and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can decrease losses throughout the power
path. If extremely thin high-resistance inductors are
used, as is common for LCD-panel applications, the
best LIR can increase to between 0.5 and 1.0.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions.
Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current
(I
MAIN(MAX)
), the expected efficiency (η
TYP
) taken from
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current at the minimum input voltage V
IN(MIN)
using conservation of energy and the expected efficiency at that
operating point (η
MIN
) taken from an appropriate curve
in the Typical Operating Characteristics:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX1790/MAX8715s’ LX current limit (I
LIM
) should
exceed I
PEAK
and the inductor’s DC current rating should
exceed I
IN(DC,MAX)
. For good efficiency, choose an
inductor with less than 0.1Ω series resistance.
Considering the application circuit in Figure 4, the maxi-
mum load current (I
MAIN(MAX)
) is 150mA with a 9V output
and a typical input voltage of 3.3V. Choosing an LIR of 0.5
and estimating efficiency of 85% at this operating point:
Using the circuit’s minimum input voltage (3V) and estimating efficiency of 80% at that operating point:
The ripple current and the peak current are:
Diode Selection
The output diode should be rated to handle the output
voltage and the peak switch current. Make sure that the
diode’s peak current rating is at least IPKand that its
breakdown voltage exceeds V
OUT
. Schottky diodes are
recommended.
Input and Output Capacitor Selection
Low-ESR capacitors are recommended for input
bypassing and output filtering. Low-ESR tantalum
capacitors are a good compromise between cost and
performance. Ceramic capacitors are also a good
choice. Avoid standard aluminum electrolytic capacitors. A simple equation to estimate input and outputcapacitor values for a given voltage ripple is as follows:
where V
RIPPLE
is the peak-to-peak ripple voltage on the
capacitor.
Output Voltage
The MAX1790/MAX8715 operate with an adjustable
output from VINto 13V. Connect a resistor voltagedivider to FB (see the Typical Operating Circuit) from
the output to GND. Select the resistor values as follows:
where VFB, the boost-regulator feedback set point, is
1.24V. Since the input bias current into FB is typically 0,
RR
V
V
OUT
FB
12 1=−
⎛
⎝
⎜
⎞
⎠
⎟
C
0.5 L I
V V
PK
2
RIPPLE OUT
≥
××
⎛
⎝
⎞
⎠
×
IA
A
A
PEAK
=+ ≈06
025
2
0 725.
.
.
I
VVV
H V MHz
A
RIPPLE
=
×−
××
≈
393
68 9 12
025
()
..
.
µ
I
AV
V
A
IN DC MAX(, )
.
.
.=
×
×
≈
015 9
308
06
L
V
V
VV
A MHz
H=
⎛
⎝
⎜
⎞
⎠
⎟
−
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
≈
33
9
933
015 12
085
05
68
2
..
....
. µ
II
I
PEAK IN DC MAX
RIPPLE
=+
(, )
2
I
VVV
LV f
RIPPLE
IN MIN MAIN IN MIN
MAIN OSC
=
×−
××
() ()
()
I
IV
V
IN DC MAX
MAIN MAX MAIN
IN MIN MIN
(, )
()
()
=
×
×η
L
V
V
VV
I f LIR
IN
MAIN
MAIN IN
MAIN MAX OSC
TYP
=
⎛
⎝
⎜
⎞
⎠
⎟
−
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
2
()
η
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
12 ______________________________________________________________________________________
R2 can have a value up to 100kΩ without sacrificing
accuracy. Connect the resistor-divider as close to the IC
as possible.
Loop Compensation
The voltage feedback loop needs proper compensation
to prevent excessive output ripple and poor efficiency
caused by instability. This is done by connecting a resistor (R
COMP
) and capacitor (C
COMP
) in series from
COMP to GND, and another capacitor (C
COMP2
) from
COMP to GND. R
COMP
is chosen to set the high-frequency integrator gain for fast transient response, while
C
COMP
is chosen to set the integrator zero to maintain
loop stability. The second capacitor, C
COMP2
, is chosen
to cancel the zero introduced by output-capacitance
ESR. For optimal performance, choose the components
using the following equations:
R
COMP
≅ (200Ω / A2) x V
OUT
2
xC
OUT
/ L (MAX1790)
R
COMP
≅ (274Ω / A) x VINxV
OUTxCOUT
/ (L x I
OUT
)
(MAX8715)
C
COMP
≅ (0.4 x 10-3A/Ω) x L / V
IN
(MAX1790)
C
COMP
≅ (0.36 x 10-3A/Ω) x L / V
IN
(MAX8715)
C
COMP2
≅ (0.005 A2/Ω) x R
ESR
x L / V
OUT
2
(MAX1790)
C
COMP2
≅ (0.0036 A/Ω) x R
ESR
x L x I
OUT
/ (V
IN
x
V
OUT
)
(MAX8715)
For the ceramic output capacitor, where ESR is small,
C
COMP2
is optional. Table 1 shows experimentally verified
external component values for several applications.
The best gauge of correct loop compensation is by
inspecting the transient response of the MAX1790/
MAX8715. Adjust R
COMP
and C
COMP
as necessary to
obtain optimal transient performance.
Soft-Start Capacitor
The soft-start capacitor should be large enough that it
does not reach final value before the output has
reached regulation. Calculate CSSto be:
where:
C
OUT
= total output capacitance including any bypass
capacitor on the output bus
V
OUT
= maximum output voltage
I
INRUSH
= peak inrush current allowed
I
OUT
= maximum output current during power-up stage
V
IN
= minimum input voltage
The load must wait for the soft-start cycle to finish
before drawing a significant amount of load current.
The duration after which the load can begin to draw
maximum load current is:
t
MAX
= 6.77 x 105C
SS
Application Circuits
1-Cell to 3.3V SEPIC Power Supply
Figure 3 shows the MAX1790 in a single-ended primary
inductance converter (SEPIC) topology. This topology is
useful when the input voltage can be either higher or
lower than the output voltage, such as when converting
a single lithium-ion (Li+) cell to a 3.3V output. L1A and
L1B are two windings on a single inductor. The coupling
capacitor between these two windings must be a lowESR type to achieve maximum efficiency, and must also
be able to handle high ripple currents. Ceramic capacitors are best for this application. The circuit in Figure 3
provides 400mA output current at 3.3V output when
operating with an input voltage from +2.6V to +5.5V.
C 21 10 C
V
V V
V I I V
SS
6
OUT
IN OUT
IN INRUSH OUT OUT
OUT
2
>× ×
−×
×−×
⎛
⎝
⎜
⎞
⎠
⎟
−
LX
IN
V
IN
2.6V TO 5.5V
GND
L1 = CTX8-1P
C
OUT
= TPSD226025R0200
C2
10µF
FREQ
V
OUT
3.3V
CC
SS
SHDN
FB
D1
R1
1MΩ
R2
605kΩ
L1A
5.3µH
0.027µF
MAX1790
C
OUT
22µF
20V
C1
10µF
10V
R
COMP
22kΩ
C
COMP
330pF
C
COMP2
56pF
L1B
5.3µH
Figure 3. MAX1790 in a SEPIC Configuration
AMLCD Application
Figure 4 shows a power supply for active matrix (TFTLCD) flat-panel displays. Output-voltage transient performance is a function of the load characteristic. Add or
remove output capacitance (and recalculate compensation-network component values) as necessary to
meet transient performance. Regulation performance
for secondary outputs (V2 and V3) depends on the load
characteristics of all three outputs.
Layout Procedure
Good PC board layout and routing are required in highfrequency switching power supplies to achieve good reg-
ulation, high efficiency, and stability. It is strongly recommended that the evaluation kit PC board layouts be followed as closely as possible. Place power components
as close together as possible, keeping their traces short,
direct, and wide. Avoid interconnecting the ground pins
of the power components using vias through an internal
ground plane. Instead, keep the power components
close together and route them in a star ground configuration using component-side copper, then connect the star
ground to internal ground using multiple vias.
Chip Information
TRANSISTOR COUNT: 1012
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
______________________________________________________________________________________ 13
Figure 4. Multiple-Output, Low-Profile (1.2mm max) TFT-LCD Power Supply
FB
LX
GND
18pF (MAX1790)
10pF (MAX8715)
27nF
C1, C2, C3, C4:
TAIYO YUDEN LMK325BJ335MD (3.3µF, 10V)
D1:
ZETEX ZHCS1000 (20V, 1A, SCHOTTKY) OR MOTOROLA MBRM120ET3
D2, D3, D4:
ZETEX BAT54S (30V, 200mA, SCHOTTKY)
L1:
SUMIDA CLQ4D10-6R8 (6.8µH, 0.8A) OR SUMITOMO CXLM120-6R8
274k
Ω
44.2k
Ω
FREQ
IN
3.0V TO 3.6V
V2
+26V
5mA
V1
9V
150mA
V3
-9V
10mA
COMP
SHDN
SS
D1
D2
D3
D4
0.47µF
1µF
1µF
1µF
0.1µF
0.1µF
MAX1790
MAX8715
470pF (MAX1790)
750pF (MAX8715)
150kΩ (MAX1790)
82kΩ (MAX8715)
C1
C3
C2
3.3µF
C4
L1
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
8LUMAXD.EPS
PACKAGE OUTLINE, 8L uMAX/uSOP
1
1
21-0036
J
REV.DOCUMENT CONTROL NO.APPROVAL
PROPRIETARY INFORMATION
TITLE:
MAX
0.043
0.006
0.014
0.120
0.120
0.198
0.026
0.007
0.037
0.0207 BSC
0.0256 BSC
A2
A1
c
e
b
A
L
FRONT VIEW
SIDE VIEW
E H
0.6±0.1
0.6±0.1
ÿ 0.50±0.1
1
TOP VIEW
D
8
A2
0.030
BOTTOM VIEW
1
6∞
S
b
L
H
E
D
e
c
0∞
0.010
0.116
0.116
0.188
0.016
0.005
8
4X S
INCHES
-
A1
A
MIN
0.002
0.950.75
0.5250 BSC
0.25 0.36
2.95 3.05
2.95 3.05
4.78
0.41
0.65 BSC
5.03
0.66
6∞0∞
0.13 0.18
MAX
MIN
MILLIMETERS
- 1.10
0.05 0.15
α
α
DIM
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages
.)
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