MAXIM MAX8715EUA

19-1563; Rev 3; 9/05
KIT
ATION
EVALU
LE
B
A
IL
A
AV
Low-Noise Step-Up DC-DC Converters
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.
µMAX is a registered trademark of Maxim Integrated Products,
Inc.
Applications
Features
♦ 90% Efficiency
♦ Adjustable Output from VIN 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
Ordering Information
PART
MAX1790EUA
TEMP RANGE
PIN-PACKAGE
-40°C to +85°C
8 µMAX
MAX1790EUA+
-40°C to +85°C
8 µMAX
MAX8715EUA
-40°C to +85°C
8 µMAX
PCMCIA Cards
MAX8715EUA+
-40°C to +85°C
8 µMAX
Portable Applications
+ Denotes lead-free package.
LCD Displays
Hand-Held Devices
Typical Operating Circuit
Pin Configuration
VIN
2.6V TO 5V
TOP VIEW
IN
ON/OFF
VOUT
LX
SHDN
1
FB
2
SHDN
MAX1790
MAX8715
3
MAX1790
MAX8715
GND 4
FREQ
GND
SS
COMP
8
SS
7
FREQ
6
IN
5
LX
μMAX
FB
COMP
________________________________________________________________ Maxim Integrated Products
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.
1
MAX1790/MAX8715
General Description
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
ABSOLUTE MAXIMUM RATINGS
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 (VIN + 0.3V)
RMS LX Pin Current ..............................................................1.2A
Continuous Power Dissipation (TA = +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
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
(VIN = SHDN = 3V, FREQ = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Input Supply Range
VIN Undervoltage Lockout
SYMBOL
CONDITIONS
VIN
UVLO
VIN rising, typical hysteresis is 40mV,
LX remains off below this level
IIN
MAX8715
2.25
VFB = 1.3V, not switching
VFB = 1.0V, switching
MAX
UNITS
5.5
V
2.38
2.52
V
0.18
0.35
2
5
VFB = 1.3V, not switching
0.21
0.35
VFB = 1.0V, switching
2.5
5.0
0.1
10
µA
1.24
1.258
V
IIN
SHDN = GND
Feedback Voltage
VFB
Level to produce VCOMP = 1.24V
FB Input Bias Current
IFB
VFB = 1.24V
Shutdown Supply Current
TYP
2.6
MAX1790
Quiescent Current
MIN
mA
ERROR AMPLIFIER
Feedback-Voltage Line
Regulation
1.222
MAX1790
0
40
MAX8715
125
190
0.05
0.15
Level to produce VCOMP = 1.24V,
2.6V < VIN < 5.5V
Transconductance
gm
Voltage Gain
AV
ΔI = 5µA
MAX1790
70
140
240
MAX8715
70
160
240
700
nA
%/V
µS
V/V
OSCILLATOR
Frequency
Maximum Duty Cycle
fOSC
DC
FREQ = GND
540
640
740
FREQ = IN
1000
1220
1500
79
85
92
FREQ = GND
FREQ = IN
84
kHz
%
N-CHANNEL SWITCH
Current Limit
On-Resistance
Leakage Current
2
ILIM
RON
ILXOFF
VFB = 1V,
duty cycle =
65% (Note 1)
MAX1790
1.2
1.6
2.3
MAX8715
1.8
2.4
3.4
A
MAX1790
0.21
0.5
MAX8715
0.15
0.35
MAX1790
0.01
20
MAX8715
5
30
VLX = 12V
_______________________________________________________________________________________
Ω
µA
Low-Noise Step-Up DC-DC Converters
(VIN = SHDN = 3V, FREQ = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Current-Sense Transresistance
SYMBOL
RCS
CONDITIONS
MIN
TYP
MAX
MAX1790
0.30
0.45
0.65
MAX8715
0.20
0.30
0.43
VSS = 1.2V
1.5
4
UNITS
V/A
SOFT-START
Reset Switch Resistance
Charge Current
100
Ω
7.0
µA
CONTROL INPUTS
Input Low Voltage
VIL
SHDN, FREQ
Input High Voltage
VIH
SHDN, FREQ
0.3 x VIN
0.7 x VIN
SHDN, FREQ
Hysteresis
FREQ Pulldown Current
IFREQ
SHDN Input Current
ISHDN
V
0.1 x VIN
1.8
V
V
5
9.0
µA
0.001
1
µA
TYP
MAX
UNITS
2.6
5.5
V
2.25
2.52
V
ELECTRICAL CHARACTERISTICS
(VIN = SHDN = 3V, FREQ = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
Input Supply Range
SYMBOL
CONDITIONS
VIN
VIN Undervoltage Lockout
UVLO
VIN rising, typical hysteresis is 40mV,
LX remains off below this level
MAX1790
Quiescent Current
IIN
MAX8715
Shutdown Supply Current
MIN
VFB = 1.3V, not switching
0.35
VFB = 1.0V, switching
5
VFB = 1.3V, not switching
0.35
VFB = 1.0V, switching
IIN
SHDN = GND
VFB
Level to produce VCOMP = 1.24V
mA
5
10
µA
1.260
V
ERROR AMPLIFIER
Feedback Voltage
FB Input Bias Current
IFB
Feedback-Voltage Line
Regulation
Transconductance
VFB = 1.24V
1.215
40
MAX8715
190
Level to produce VCOMP = 1.24V,
2.6V < VIN < 5.5V
gm
ΔI = 5µA
1.24
MAX1790
0.15
MAX1790
70
260
MAX8715
70
260
nA
%/V
µS
OSCILLATOR
Frequency
fOSC
Maximum Duty Cycle
DC
FREQ = GND
490
770
FREQ = IN
900
1500
FREQ = GND
78
92
kHz
%
_______________________________________________________________________________________
3
MAX1790/MAX8715
ELECTRICAL CHARACTERISTICS (continued)
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(VIN = SHDN = 3V, FREQ = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
MAX1790
1.2
2.3
MAX8715
1.8
3.0
UNITS
N-CHANNEL SWITCH
Current Limit
ILIM
On-Resistance
RON
Current-Sense Transresistance
RCS
VFB = 1V,
duty cycle =
65% (Note 1)
A
MAX1790
0.5
MAX8715
0.35
MAX1790
0.30
0.65
MAX8715
0.20
0.43
Ω
V/A
CONTROL INPUTS
Input Low Voltage
VIL
SHDN, FREQ
Input High Voltage
VIH
SHDN, FREQ
0.3 x VIN
0.7 x VIN
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.
4
_______________________________________________________________________________________
V
V
Low-Noise Step-Up DC-DC Converters
MAX1790 EFFICIENCY
vs. OUTPUT CURRENT
75
fOSC = 1.2MHz
L = 2.7μH
65
60
80
fOSC = 1.2MHz
L = 5.4μH
75
90
85
70
65
VIN = 3.3V
VOUT = 5V
55
1
10
100
VIN = 3.3V
VOUT = 12V
55
50
1000
fOSC = 1.2MHz
L = 5.4μH
75
70
65
1
10
100
VIN = 5V
VOUT = 12V
55
50
1
1000
10
100
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1790 OUTPUT VOLTAGE
vs. OUTPUT CURRENT
MAX8715 EFFICIENCY
vs. OUTPUT CURRENT
0.4
fOSC = 1.2MHz
0.3
0.2
0.1
11.90
TA = +25°C
11.85
TA = -40°C
11.80
3.5
4.0
4.5
INPUT VOLTAGE (V)
5.0
5.5
1000
MAX1790 toc06
85
80
75
70
65
60
11.70
55
fOSC = 640kHz
VOUT = 9V
fOSC = 1.2MHz
L = 6.8μH
90
11.75
11.60
3.0
95
11.95
11.65
VOUT = 12V
0
12.00
OUTPUT VOLTAGE (V)
fOSC = 640kHz
0.5
TA = +85°C
EFFICIENCY (%)
0.6
12.05
MAX1790 toc05
12.10
MAX1790 toc04
0.7
2.5
fOSC = 640kHz
L = 10μH
80
60
60
50
NO-LOAD SUPPLY CURRENT (mA)
fOSC = 640kHz
L = 10μH
MAX1790 toc03
85
EFFICIENCY (%)
80
90
EFFICIENCY (%)
EFFICIENCY (%)
85
95
MAX1790 toc02
fOSC = 640kHz
L = 5.4μH
70
95
MAX1790 toc01
95
90
MAX1790 EFFICIENCY
vs. OUTPUT CURRENT
MAX1790 EFFICIENCY
vs. OUTPUT CURRENT
VIN = 5.0V
VIN = 3.3V
50
45
0
20 40 60 80 100 120 140 160 180 200
OUTPUT CURRENT (mA)
1
10
100
1000
OUTPUT CURRENT (mA)
_______________________________________________________________________________________
5
MAX1790/MAX8715
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 3.3V, fOSC = 640kHz, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 3.3V, fOSC = 640kHz, TA = +25°C, unless otherwise noted.)
MAX8715
PULSED LOAD-TRANSIENT RESPONSE
10mA
RCOMP = 82kΩ
CCOMP = 750pF
CCOMP2 = 10pF
1A
CH1
40mA
200mA
CH1
10mA
CH2
RCOMP = 120kΩ
CCOMP = 1200pF
CCOMP2 = 56pF
MAX1790 toc09
MAX1790 LOAD-TRANSIENT RESPONSE
MAX1790 toc08
200mA
CH1
0
MAX1790 toc07
MAX8715 LOAD-TRANSIENT RESPONSE
CH2
CH2
CH3
CH3
CH3
40μs/div
MAX1790 toc12
CH1
CH1
CH2
CH2
CH2
CH3
CH3
CH3
100μs/div
CH1 = LOAD CURRENT, 500mA/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
VOUT = 5V, fOSC = 640kHz, COUT = 47μF + 0.1μF
6
STARTUP WAVEFORM
WITH SOFT-START
MAX1790 STARTUP WAVEFORM
WITHOUT SOFT-START
MAX1790 toc11
RCOMP = 62kΩ
CCOMP = 820pF
CCOMP2 = 56pF
CH1 = LOAD CURRENT, 100mA/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
VIN = 3V
VOUT = 12V, fOSC = 640kHz, COUT = 33μF + 0.1μF
CH1 = LOAD CURRENT, 1A/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 100mV/div
CH3 = INDUCTOR CURRENT, 500mA/div
VIN = 3.3V, VOUT = 9.0V
fOSC = 1.2MHz, L = 6.8μH, COUT = 3 x 3.3μF
MAX1790 LOAD-TRANSIENT RESPONSE
500mA
CH1
20mA
100μs/div
10μs/div
CH1 = LOAD CURRENT, 200mA/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 100mV/div
CH3 = INDUCTOR CURRENT, 500mA/div
VIN = 3.3V, VOUT = 9.0V
fOSC = 1.2MHz, L = 6.8μH, COUT = 3 x 3.3μF
MAX1790 toc10
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
100μs/div
CH1 = SHDN, 5V/div
CH2 = OUTPUT VOLTAGE, 5V/div
CH3 = INDUCTOR CURRENT, 1A/div
VIN = 3.3V, VOUT = 12V, IOUT = 10mA, fOSC = 640kHz
NO SOFT-START CAPACITOR, COUT = 33μF
1ms/div
CH1 = SHDN, 5V/div
CH2 = OUTPUT VOLTAGE, 5V/div
CH3 = INDUCTOR CURRENT, 200mA/div
VOUT = 12V, IOUT = 10mA, fOSC = 640kHz,
CSS = 0.027μF, COUT = 33μF
_______________________________________________________________________________________
Low-Noise Step-Up DC-DC Converters
STARTUP WAVEFORM
WITH SOFT-START
MAX1790 toc13
MAX1790 toc14
SWITCHING WAVEFORM
CH1
CH1
CH2
CH2
CH3
CH3
500ns/div
2ms/div
CH1 = LX SWITCHING WAVEFORM, 5V/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
VOUT = 12V, IOUT = 200mA, fOSC = 640kHz, L = 10μH;
COUT = 33μF + 0.1μF
CH1 = SHDN, 5V/div
CH2 = VOUT, 5V/div
CH3 = INDUCTOR CURRENT, 500mA/div
VOUT = 12V, IOUT = 200mA, fOSC = 640kHz,
CSS = 0.027μF
1600
1400
VOUT = 5V
1200
1000
800
600
VOUT = 12V
400
1800
MAXIMUM OUTPUT CURRENT (mA)
MAX1790 toc15
MAXIMUM OUTPUT CURRENT (mA)
1800
1600
1400
VOUT = 9V
fOSC = 1.2MHz
L = 6.8μH
COUT = 3 x 3.3μF
MAX1790 toc16
MAX8715 MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
MAX1790 MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
1200
1000
800
600
400
200
200
fOSC = 640kHz
0
0
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
_______________________________________________________________________________________
7
MAX1790/MAX8715
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 3.3V, fOSC = 640kHz, TA = +25°C, unless otherwise noted.)
Low-Noise Step-Up DC-DC Converters
MAX1790/MAX8715
Pin Description
PIN
NAME
1
COMP
2
FB
3
SHDN
Shutdown Control Input. Drive SHDN low to turn off the MAX1790/MAX8715.
4
GND
Ground
5
LX
Switch Pin. Connect the inductor/catch diode to LX and minimize the trace area for lowest EMI.
6
IN
Supply Pin. Bypass IN with at least a 1µF ceramic capacitor directly to GND.
7
FREQ
8
SS
FUNCTION
Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop
Compensation section for component selection guidelines.
Feedback Pin. Reference voltage is 1.24V nominal. Connect an external resistor-divider tap to FB and
minimize the trace area. Set VOUT according to: VOUT = 1.24V (1 + R1 / R2). See Figure 1.
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.
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 105 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.
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.
VIN
2.6V TO 5.5V
CIN
C1
10μF
6.3V
L
IN
ON/OFF
VOUT
LX
SHDN
VIN
D1
MBRS130LT1
0.1μF*
MAX1790
MAX8715
1.2MHz
GND
640kHz
SS
0.027μF
FB
R1
COMP
R2
CCOMP2
RCOMP
CCOMP
Figure 1. Typical Application Circuit
8
COUT
FREQ
_______________________________________________________________________________________
* OPTIONAL
Low-Noise Step-Up DC-DC Converters
4μA
MAX1790/MAX8715
SKIP
COMPARATOR
SHDN
IN
BIAS
SOFTSTART
SKIP
COMP
ERROR
AMPLIFIER
SS
ERROR
COMPARATOR
FB
∞
LX
CONTROL
AND DRIVER
LOGIC
1.24V
N
CLOCK
GND
OSCILLATOR
FREQ
SLOPE
COMPENSATION
Σ
CURRENT
SENSE
5μA
MAX1790
MAX8715
Figure 2. Functional Diagram
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:
IOUT(MAX) = [ILIM x (1.26 - 0.4 x Duty) 0.5 x Duty x VIN / (fOSC x L)] x η x VIN / VOUT
where:
ILIM = current limit specified at 65% (see the Electrical
Characteristics)
Duty = duty cycle = (VOUT - VIN + VDIODE) /
(VOUT - ILIM x RON + VDIODE)
VDIODE = catch diode forward voltage at ILIM
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 (CSS)
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 VSS = 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
_______________________________________________________________________________________
9
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
Table 1. Component Selection
VOUT
(V)
fOSC
(Hz)
L (µH)
COUT (µF)
RCOMP
(kΩ)
CCOMP
(pF)
CCOMP2
(pF)
IOUT(MAX)
(mA)
3.3
12
640k
10 (Sumida
CDRH5D18-100NC)
33 tantalum (AVX
TPSD336020R0200)
120
1200
22
250
3.3
12
1.2M
5.4 (Sumida
CDRH5D18-5R4NC)
33 tantalum (AVX
TPSD336020R0200)
180
650
20
250
3.3
5
640k
5.4 (Sumida
CDRH5D18-5R4NC)
47 tantalum
(6TPA47M)
62
820
56
800
3.3
5
1.2M
2.7 (Sumida
CDRH4D18-2R7)
47 tantalum
(6TPA47M)
91
390
33
800
9
1.2M
6.8 (Sumida
CLQ4D10-6R8)
3 x 3.3 ceramic
(Taiyo Yuden
LMK325BJ335MD)
82
750
10
150
VIN (V)
MAX1790
MAX8715
3.3
Table 2. Component Suppliers
SUPPLIER
PHONE
FAX
Coilcraft
847-639-6400
847-639-1469
Coiltronics
561-241-7876
561-241-9339
Sumida USA
847-956-0666
847-956-0702
TOKO
847-297-0070
847-699-1194
AVX
803-946-0690
803-626-3123
Kemet
408-986-0424
408-986-1442
Sanyo
619-661-6835
619-661-1055
Taiyo Yuden
408-573-4150
408-573-4159
Central
Semiconductor
516-435-1110
516-435-1824
International
Rectifier
310-322-3331
310-322-3332
Motorola
602-303-5454
602-994-6430
Nihon
847-843-7500
847-843-2798
Zetex
516-543-7100
516-864-7630
Inductor Selection
Inductors
Capacitors
Diodes
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,
10
as well as maximum and minimum input voltages.
Begin by selecting an inductor value. Once L is known,
choose the diode and capacitors.
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
______________________________________________________________________________________
Low-Noise Step-Up DC-DC Converters
Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current
(IMAIN(MAX)), the expected efficiency (ηTYP) taken from
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
2
⎞⎛ η
⎛ V
⎞ ⎛
VMAIN − VIN
TYP ⎞
L = ⎜ IN ⎟ ⎜
⎟
⎟⎜
⎝ VMAIN ⎠ ⎝ IMAIN(MAX) × fOSC ⎠ ⎝ LIR ⎠
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current at the minimum input voltage VIN(MIN) using conservation of energy and the expected efficiency at that
operating point (ηMIN) taken from an appropriate curve
in the Typical Operating Characteristics:
IIN(DC,MAX) =
IMAIN(MAX) × VMAIN
VIN(MIN) × ηMIN
Calculate the ripple current at that operating point and
the peak current required for the inductor:
IRIPPLE =
VIN(MIN) × (VMAIN − VIN(MIN) )
L × VMAIN × fOSC
I
IPEAK = IIN(DC,MAX) + RIPPLE
2
The inductor’s saturation current rating and the
MAX1790/MAX8715s’ LX current limit (ILIM ) should
exceed IPEAK and the inductor’s DC current rating should
exceed IIN(DC,MAX). For good efficiency, choose an
inductor with less than 0.1Ω series resistance.
Considering the application circuit in Figure 4, the maximum load current (IMAIN(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:
2
9V − 3.3V ⎞ ⎛ 0.85 ⎞
⎛ 3.3V ⎞ ⎛
L=⎜
⎟ ⎜
⎟⎜
⎟ ≈ 6.8μH
⎝ 9V ⎠ ⎝ 0.15A × 1.2MHz ⎠ ⎝ 0.5 ⎠
Using the circuit’s minimum input voltage (3V) and estimating efficiency of 80% at that operating point:
IIN(DC,MAX) =
0.15A × 9V
≈ 0.6A
3V × 0.8
The ripple current and the peak current are:
IRIPPLE =
3V × (9V − 3V)
≈ 0.25A
6.8μH × 9V × 1.2MHz
IPEAK = 0.6A +
0.25A
≈ 0.725A
2
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 IPK and that its
breakdown voltage exceeds VOUT. 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:
0.5 × L × ⎛⎝IPK 2 ⎞⎠
C≥
VRIPPLE × VOUT
where VRIPPLE is the peak-to-peak ripple voltage on the
capacitor.
Output Voltage
The MAX1790/MAX8715 operate with an adjustable
output from VIN to 13V. Connect a resistor voltagedivider to FB (see the Typical Operating Circuit) from
the output to GND. Select the resistor values as follows:
⎛V
⎞
R1 = R2 ⎜ OUT − 1⎟
⎝ VFB
⎠
where VFB, the boost-regulator feedback set point, is
1.24V. Since the input bias current into FB is typically 0,
______________________________________________________________________________________
11
MAX1790/MAX8715
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.
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
R2 can have a value up to 100kΩ without sacrificing
accuracy. Connect the resistor-divider as close to the IC
as possible.
VIN
2.6V TO 5.5V
C1
10μF
10V
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 (CCOMP2) from
COMP to GND. RCOMP is chosen to set the high-frequency integrator gain for fast transient response, while
CCOMP is chosen to set the integrator zero to maintain
loop stability. The second capacitor, CCOMP2, is chosen
to cancel the zero introduced by output-capacitance
ESR. For optimal performance, choose the components
using the following equations:
RCOMP ≅ (200Ω / A2) x VOUT2 x COUT / L (MAX1790)
RCOMP ≅ (274Ω / A) x VIN x VOUT x COUT / (L x IOUT)
(MAX8715)
CCOMP ≅ (0.4 x 10 -3 A/Ω) x L / VIN
CCOMP ≅ (0.36 x 10 -3 A/Ω) x L / VIN
(MAX1790)
(MAX8715)
CCOMP2 ≅ (0.005 A2/Ω) x RESR x L / VOUT2
(MAX1790)
CCOMP2 ≅ (0.0036 A/Ω) x RESR x L x IOUT / (VIN x VOUT)
(MAX8715)
For the ceramic output capacitor, where ESR is small,
CCOMP2 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 RCOMP and CCOMP 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 CSS to be:
⎞
⎛
VOUT 2 − VIN × VOUT
CSS > 21 × 10 −6 × COUT ⎜
⎟
⎝ VIN × IINRUSH − IOUT × VOUT ⎠
where:
COUT = total output capacitance including any bypass
capacitor on the output bus
VOUT = maximum output voltage
IINRUSH = peak inrush current allowed
12
L1A
5.3μH
C2
10μF
IN
D1
LX
SHDN
L1B
5.3μH
MAX1790
FREQ
VOUT
3.3V
COUT
22μF
20V
GND
SS
0.027μF
FB
CC
R2
605kΩ
CCOMP2
56pF
R1
1MΩ
RCOMP
22kΩ
CCOMP
330pF
L1 = CTX8-1P
COUT = TPSD226025R0200
Figure 3. MAX1790 in a SEPIC Configuration
IOUT = maximum output current during power-up stage
VIN = 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:
tMAX = 6.77 x 105 CSS
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.
______________________________________________________________________________________
Low-Noise Step-Up DC-DC Converters
0.1μF
D3
3.3μF
D4
0.1μF
MAX1790/MAX8715
D2
V2
+26V
5mA
V3
-9V
10mA
1μF
1μF
1μF
D1
3.0V TO 3.6V
C1
0.47μF
L1
C2
C3
C4
V1
9V
150mA
274kΩ
LX
FB
IN
MAX1790
MAX8715
44.2kΩ
FREQ
SHDN
GND
COMP
SS
150kΩ (MAX1790)
82kΩ (MAX8715)
470pF (MAX1790)
750pF (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
18pF (MAX1790)
10pF (MAX8715)
Figure 4. Multiple-Output, Low-Profile (1.2mm max) TFT-LCD Power Supply
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
______________________________________________________________________________________
13
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.)
4X S
8
8
INCHES
DIM
A
A1
A2
b
E
Ø0.50±0.1
H
c
D
e
E
H
0.6±0.1
L
1
1
α
0.6±0.1
S
BOTTOM VIEW
D
MIN
0.002
0.030
MAX
0.043
0.006
0.037
0.014
0.010
0.007
0.005
0.120
0.116
0.0256 BSC
0.120
0.116
0.198
0.188
0.026
0.016
6∞
0∞
0.0207 BSC
8LUMAXD.EPS
MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
MILLIMETERS
MAX
MIN
0.05
0.75
1.10
0.15
0.95
0.25
0.36
0.13
0.18
2.95
3.05
0.65 BSC
2.95
3.05
5.03
4.78
0.41
0.66
0∞
6∞
0.5250 BSC
TOP VIEW
A1
A2
A
α
c
e
b
L
SIDE VIEW
FRONT VIEW
PROPRIETARY INFORMATION
TITLE:
PACKAGE OUTLINE, 8L uMAX/uSOP
APPROVAL
DOCUMENT CONTROL NO.
21-0036
REV.
J
1
1
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
© 2005 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.