Page 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
General Description
The MAX1790 boost converter incorporates high-performance (at 1.2MHz), current-mode, fixed-frequency, pulsewidth modulation (PWM) circuitry with a built-in 0.21Ω
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 is
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
♦ +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
Low-Noise Step-Up DC-DC Converter
________________________________________________________________ Maxim Integrated Products 1
Typical Operating Circuit
19-1563; Rev 0; 1/00
PART
MAX1790EUA -40°C to +85°C
TEMP. RANGE PIN-PACKAGE
8 µMAX
EVALUATION KIT MANUAL
FOLLOWS DATA SHEET
Pin Configuration
Ordering Information
V
IN
2.6V TO 5V
ON/OFF
TOP VIEW
1
IN
SHDN
FREQ
SS
LX
MAX1790
GND
FB
COMP
V
OUT
COMP
SHDN
2
3
4
87SS
MAX1790
µMAX
FREQFB
IN
6
LXGND
5
Page 2
MAX1790
Low-Noise Step-Up DC-DC Converter
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 +6V
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 ................................................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
VSS= 1.2V
VFB= 1.3V, not switching
VLX= 12V
VINrising, typical hysteresis is 40mV,
LX remains off below this level
ILX= 1.2A
VFB= 1V, duty cycle = 65%
FREQ = IN
∆I = 5µA
FREQ = GND
FREQ = IN
Level to produce V
COMP
= 1.24V,
2.6V < V
IN
< 5.5V
VFB= 1.24V
VFB= 1.0V, switching
SHDN = GND
FREQ = GND
Level to produce V
COMP
= 1.24V
CONDITIONS
µA
1.5 4 7
Charge Current
Ω
100
Reset Switch Resistance
V/A
0.3 0.45 0.65
R
CS
Current-Sense Transresistance
µA
0.01 20
I
LXOFF
Leakage Current
Ω
0.21 0.5
R
ON
On-Resistance
A
1.2 1.6 2.3
I
LIM
Current Limit (Note 1)
84
%
79 85 92
DCMaximum Duty Cycle
1000 1220 1500
kHz
540 640 740
f
OSC
Frequency
mA
0.18 0.35
I
IN
Quiescent Current
V
2.25 2.38 2.52
UVLO
V
2.6 5.5
V
IN
Input Supply Range
VINUndervoltage Lockout
V/V
700
A
V
Voltage Gain
µmhos
70 140 240
g
m
Transconductance
%/V
0.05 0.15
Feedback-Voltage Line
Regulation
nA
040
I
FB
FB Input Bias Current
25
µA
0.1 10
I
IN
Shutdown Supply Current
V
1.222 1.24 1.258
V
FB
Feedback Voltage
UNITSMIN TYP MAXSYMBOLPARAMETER
SHDN, FREQ
SHDN, FREQ; VIN= 2.6V to 5.5V
SHDN, FREQ; VIN= 2.6V to 5.5V
µA
0.001 1
I
SHDN
SHDN Input Current
µA
1.8 5 9
I
FREQ
FREQ Pull-Down Current
V
0.1 · V
IN
Hysteresis
V
0.7 · V
IN
V
IH
Input High Voltage
V
0.3 · V
IN
V
IL
Input Low Voltage
ERROR AMPLIFIER
OSCILLATOR
N-CHANNEL SWITCH
SOFT-START
CONTROL INPUTS
Page 3
MAX1790
Low-Noise Step-Up DC-DC Converter
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS
(VIN= SHDN = 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
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.
PARAMETER SYMBOL MIN TYP MAX UNITS
Feedback Voltage V
FB
1.215 1.26
V
Shutdown Supply Current I
IN
10
µA
45
FB Input Bias Current I
FB
40
nA
Feedback-Voltage Line
Regulation
0.15
%/V
Transconductance g
m
70 260
µmhos
V
IN
Undervoltage Lockout
Input Supply Range V
IN
2.6 5.5
V
UVLO
2.25 2.52
V
Quiescent Current I
IN
0.2 0.35
mA
Frequency f
OSC
490 770
kHz
900 1500
Maximum Duty Cycle DC
78 92
%
Current Limit I
LIM
1.2 2.3
A
On-Resistance R
ON
0.5
Ω
Current-Sense Transresistance R
CS
0.3 0.65
V/A
Input Low Voltage V
IL
0.3 · V
IN
V
Input High Voltage V
IH
0.7 · V
IN
V
CONDITIONS
FREQ = GND
Level to produce V
COMP
= 1.24V
FREQ = IN
SHDN = GND
VFB= 1.0V, switching
VFB= 1.24V
Level to produce V
COMP
= 1.24V,
2.6V < VIN< 5.5V
FREQ = GND
∆I = 5µA
VFB= 1V, duty cycle = 65%
ILX= 1.2A
V
IN
rising, typical hysteresis is 40mV,
LX remains off below this level
SHDN, FREQ, VIN= 2.6V to 5.5V
SHDN, FREQ, VIN= 2.6V to 5.5V
VFB= 1.3V, not switching
ERROR AMPLIFIER
OSCILLATOR
N-CHANNEL SWITCH
CONTROL INPUTS
Page 4
MAX1790
Low-Noise Step-Up DC-DC Converter
4 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 1, VIN= 3.3V, f
OSC
= 640kHz, TA= +25°C, unless otherwise noted.)
MAX1790-01
50
1 100010010
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
EFFICIENCY vs. OUTPUT CURRENT
65
55
85
75
95
70
60
90
80
MAX1790-02
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
EFFICIENCY vs. OUTPUT CURRENT
65
55
85
75
95
70
60
90
80
MAX1790-03
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
MAX1790-04
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
0 40608020 100 120 140 180160 200
OUTPUT VOLTAGE vs. OUTPUT CURRENT
MAX1790-05
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
f
OSC
= 640kHz
TA = +85°C
TA = +25°C
TA = -40°C
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
LOAD-TRANSIENT RESPONSE
MAX1790-06
R
COMP
= 120kΩ
C
COMP
= 1200pF
C
COMP2
= 56pF
Page 5
MAX1790
Low-Noise Step-Up DC-DC Converter
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= 3.3V, f
OSC
= 640kHz, TA= +25°C, unless otherwise noted.)
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
LOAD-TRANSIENT RESPONSE
MAX1790-07
CH1
CH2
CH3
R
COMP
= 62kΩ
C
COMP
= 820pF
C
COMP2
= 56pF
100µs/div
STARTUP WAVEFORM WITHOUT
SOFT-START
MAX1790-08
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
MAX1790-09
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
CH1
CH2
CH3
STARTUP WAVEFORM WITH
SOFT-START
2ms/div
CH1 = SHDN, 5V/div
CH2 = V
CH3 = INDUCTOR CURRENT, 500mA/div
V
C
= 12V, I
OUT
= 0.027µF
SS
OUT,
5V/div
OUT
= 200mA, f
OSC
= 640kHz,
SWITCHING WAVEFORM
MAX1790-10
CH1
CH2
CH3
500ns/div
CH1 = LX SWITCHING WAVEFORM, 5V/div
CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div
CH3 = INDUCTOR CURRENT, 1A/div
= 12V, I
V
OUT
= 33µF + 0.1µF
C
OUT
= 200mA, f
OUT
= 640kHz, L = 10µH;
OSC
1800
MAX1790-11
1600
1400
1200
1000
800
600
400
MAXIMUM OUTPUT CURRENT (mA)
200
0
3.0 3.4 3.6 3.83.2 4.0 4.2 4.4 4.84.6 5.0
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
V
= 5V
OUT
V
= 12V
OUT
f
= 640kHz
OSC
INPUT VOLTAGE (V)
MAX1790-12
Page 6
Detailed Description
The MAX1790 is a highly efficient power supply that
employs a current-mode, fixed-frequency pulse-width
modulation (PWM) architecture for fast transient
response and low-noise 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 current-sense signal.
At light loads, this architecture allows the MAX1790 to
“skip” cycles to prevent overcharging the output voltage. In this region of operation, the inductor ramps up
to a peak value of about 50mA, discharges to the output, and waits until another pulse is needed again.
MAX1790
Low-Noise Step-Up DC-DC Converter
6 _______________________________________________________________________________________
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 pull-down 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
· 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. 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
IN
C
SHDN
FREQ
SS
COMP2
MAX1790
COMP
R
C
LX
GND
FB
COMP
COMP
C
10µF
L
MBRS130LT1
0.1µF*
R2
IN
C1
10µF
10V
6.3V
D1
R1
* OPTIONAL
V
OUT
C
OUT
Page 7
MAX1790
Low-Noise Step-Up DC-DC Converter
_______________________________________________________________________________________ 7
Output Current Capability
The output current capability of the MAX1790 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
· (1.26 - 0.4 · Duty) -
0.5 · Duty · V
IN
/ (f
OSC
· L)] · η · VIN/ V
OUT
where:
I
LIM
= current limit specified at 65% (see Electrical
Characteristics)
Duty = duty cycle = (V
OUT
- VIN+ V
DIODE
) /
(V
OUT
- I
LIM
· RON+ V
DIODE
)
V
DIODE
= catch diode forward voltage at I
LIM
η =conversion efficiency, 85% nominal
Soft-Start
The MAX1790 can be programmed for soft-start 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’s frequency can be user selected to
operate at either 640kHz or 1.2MHz. Tie FREQ to GND
for 640kHz operation. For a 1.2MHz switching frequency, tie FREQ to IN. This allows the use of small, minimum-height external components while maintaining low
output noise. FREQ has an internal pull-down, allowing
the user the option of leaving FREQ unconnected for
640kHz operation.
Shutdown
The MAX1790 shuts 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 via
the external inductor and catch diode.
Applications Information
Boost DC-DC converters using the MAX1790 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 components for a range of standard applications. Table 2 lists
component suppliers.
SHDN
Figure 2. Functional Diagram
COMP
FREQ
ENABLE
COMPARATOR
BIAS
ENABLE
ERROR
AMPLIFIER
FB
∞
1.24V
SLOPE
OSCILLATOR
COMPEN-
SATION
Σ
ERROR
COMPARATOR
CONTROL
AND DRIVER
LOGIC
CLOCK
CURRENT
SENSE
4µA
SOFTSTART
N
IN
SS
LX
GND
5µA
MAX1790
Page 8
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
Inductor selection depends on input voltage, output
voltage, maximum current, switching frequency, size,
and availability of inductor values. Other factors can
include efficiency and ripple voltage. Inductors are
specified by their inductance (L), peak current (I
PK
),
and resistance (Lr). The following boost-circuit equations are useful in choosing the inductor values based
on the application. They allow the trading of peak current and inductor value while allowing for consideration
of component availability and cost.
The equation used here includes a constant LIR, which
is the ratio of the inductor peak-peak AC current to
maximum average DC inductor current. A good compromise between size of the inductor and loss and output ripple is to choose an LIR of 0.3 to 0.5. The peak
inductor current is then given by:
The inductance value is then given by:
Considering the typical application circuit, the maximum DC load current (I
OUT(MAX)
) is 500mA with a 5V
output. The inductance value is then chosen to be
5.4µH, based on the above equations and using 85%
efficiency and a 640kHz operating frequency. The
inductor saturation current rating should be greater
than IPK. The resistance of the inductor windings
should be less than 0.5Ω. To minimize radiated noise in
sensitive applications, use a shielded inductor.
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
Low-Noise Step-Up DC-DC Converter
8 _______________________________________________________________________________________
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
MAX1790
V
IN
(V)
f
OSC
(Hz)
R
COMP
(kΩ)
C
COMP2
(pF)
3.3 1.2M 91 33
3.3 640k 62 56
3.3
3.3 640k 120 33
1.2M 180 20
TYPICAL
I
OUT(MAX)
(mA)
800
800
250
250
C
COMP
(pF)
C
OUT
(µF)
390
820
47 tantalum
(6TPA47M)
47 tantalum
(6TPA47M)
1200
650
L
(µH)
2.7 (Sumida
CDRH4018-2R7)
5.4 (Sumida
CDRH5D18-5R4NC)
10 (Sumida
CDRH5D18-100NC)
5.4 (Sumida
CDRH5D18-5R4NC)
V
OUT
(V)
33 tantalum (AVX
TPSD336020R0200)
5
33 tantalum (AVX
TPSD336020R0200)
5
12
12
Inductors
Capacitors
Diodes
L
IV
()
OUT(MAX) OUT
I
=
PK
()
=
V LIR I f
η
VVV
IN(MIN)
2
⋅⋅ ⋅
OUT
⋅
V
IN(MIN)
2
⋅⋅
η
⋅
()
OUT IN(MIN)
OUT(MAX) OSC
1
⋅
LIR
+
2
−
Page 9
MAX1790
Low-Noise Step-Up DC-DC Converter
_______________________________________________________________________________________ 9
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. Sanyo OS-CON types are also recommended
for their low ESR. Avoid standard aluminum electrolytic
capacitors. A simple equation to estimate input and
output capacitor 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 operates with an adjustable output from
V
IN
to 12V. Connect a resistor voltage divider to FB
(Typical Operating Circuit) from the output to GND.
Select the resistor values as follows:
where V
FB
, the boost-regulator feedback set point, is
1.24V. Since the input bias current into FB is typically 0,
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) · V
OUT
2
· C
OUT
/ L
C
COMP
≅ (0.4 · 10-3A / Ω) L / V
IN
C
COMP2
≅ (0.005 A2/ Ω) R
ESR
· L / V
OUT
2
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.
Adjust R
COMP
and C
COMP
as necessary to obtain opti-
mal 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
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:
t
MAX
= 6.77 · 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.
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
C 21 10 C
V
V V
V I I V
SS
6
OUT
OUT
2
IN OUT
IN INRUSH OUT OUT
>
−
−
⋅⋅
⋅
⋅⋅
−
0.5 L I
C
≥
V V
RIPPLE OUT
⋅⋅
⋅
PK
2
V
RR
12 1=−
OUT
V
FB
Page 10
MAX1790
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
regulation, 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
coper, then connect the star ground to internal ground
using multiple vias.
Chip Information
TRANSISTOR COUNT: 1012
Low-Noise Step-Up DC-DC Converter
10 ______________________________________________________________________________________
Figure 3. MAX1790 in a SEPIC Configuration
V
0.027µF
IN
2.6V TO 5.5V
C
COMP2
56pF
IN
SHDN
MAX1790
FREQ
SS
CC
GND
R
COMP
22k
C
330pF
LX
FB
COMP
L1A
5.3µH
10µF
R2
605k
C2
5.3µH
L1B
L1 = CTX8-1P
C
C1
10µF
10V
D1
C
OUT
22µF
20V
R1
1M
= TPSD226025R0200
OUT
V
3.3V
OUT
Page 11
MAX1790
Low-Noise Step-Up DC-DC Converter
______________________________________________________________________________________ 11
Figure 4. Multiple-Output, Low-Profile (1.2mm max) TFT LCD Power Supply
3.0V TO 3.6V
V2
+26V
5mA
1µF
C1
0.47µF
150k
470pF
D2
1µF
18pF
D3
L1
IN
FREQ
SHDN
COMP
0.1µF
0.1µF
MAX1790
LX
GND
V3
-9V
10mA
D4
1µF
D1
274k
FB
44.2k
SS
27nF
3.3µF
V1
9V
C2
C3
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
C4
150mA
Page 12
MAX1790
Low-Noise Step-Up DC-DC Converter
12 ______________________________________________________________________________________
Package Information
8LUMAXD.EPS
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