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General Description
The MAX5025–MAX5028 constant-frequency, pulsewidth modulating (PWM), low-noise boost converters
are intended for low-voltage systems that often need a
locally generated high voltage. These devices are
capable of generating low-noise, high output voltages
required for varactor diode biasing in TV tuners, set-top
boxes, and PCI cable modems. The MAX5025–
MAX5028 operate from as low as 3V and switch at
500kHz.
The constant-frequency, current-mode PWM architecture provides for low output noise that is easy to filter. A
40V lateral DMOS device is used as the internal power
switch, making the devices ideal for boost converters
up to 36V. The MAX5025/MAX5026 adjustable versions
require the use of external feedback resistors to set the
output voltage. The MAX5027/MAX5028 offer a fixed
30V output. These devices are available in a small, 6pin SOT23 package.
Applications
TV Tuner Power Supply
Low-Noise Varactor Diode Biasing
Set-Top Box Tuner Power Supply
PCI Cable Modem
Voice-Over-Cable
LCD Power Supply
Avalanche Photodiode Biasing
Features
♦ Input Voltage Range:
3V to 11V (MAX5026/MAX5028)
4.5V to 11V (MAX5025/MAX5027)
♦ Wide Output Voltage Range: V
CC
to 36V
♦ Output Power: 120mW (max)
♦ User-Adjustable Output Voltage with
MAX5025/MAX5026 Using External Feedback
Resistors
♦ Fixed 30V Output Voltage: MAX5027/MAX5028
♦ Internal 1.3Ω (typ), 40V Switch
♦ Constant PWM Frequency Provides Easy Filtering
in Low-Noise Applications
♦ 500kHz (typ) Switching Frequency
♦ 1µA (max) Shutdown Current
♦ Small, 6-Pin SOT23 Package
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
________________________________________________________________ Maxim Integrated Products 1
Pin Configuration
V
OUT
30V
C1 C2
L1
VCC = 4.5V TO 11V
(MAX5027)
V
CC
= 3V TO 11V
(MAX5028)
D1
V
CC
GND
FB
PGND
LX
MAX5027
MAX5028
SHDN
Typical Operating Circuit
19-2239; Rev 1; 12/01
EVALUATION KIT
AVAILABLE
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
Selector Guide appears at end of data sheet.
PA RT
MAX5025EUT-T -40°C to +85°C 6 SOT23-6 AATJ
MAX5026EUT-T -40°C to +85°C 6 SOT23-6 AATK
MAX5027EUT-T -40°C to +85°C 6 SOT23-6 AATL
MAX5028EUT-T -40°C to +85°C 6 SOT23-6 AATM
TEMP.
RANGE
PIN-
PA CKA GE
TOP
MARK
TOP VIEW
16LX
PGND
GND
MAX5025–
2
MAX5028
FB
34
SOT23-6
5V
CC
SHDN
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC= 5V, SHDN = VCC, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.) (Note 1)
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.
VCCto GND............................................................-0.3V to +12V
PGND to GND .......................................................-0.1V to +0.1V
FB to GND (MAX5025/MAX5026)...............-0.3V to (V
CC
+ 0.3V)
FB to GND (MAX5027/MAX5028)...........................-0.3V to +40V
SHDN to GND.............................................-0.3V to (V
CC
+ 0.3V)
LX to GND ..............................................................-0.3V to +45V
Peak LX Current ................................................................600mA
Operating Temperature Range ...........................-40°C to +85°C
Continuous Power Dissipation (T
A
= +70°C)
6-Pin SOT23 (derate 7.1mW/°C above +70°C)..........695.7mW
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +165°C
Lead Temperature (soldering 10s) ..................................+300°C
PARAMETER
CONDITIONS
UNITS
SUPPLY VOLTAGE
MAX5026/MAX5028 3.0 11
Input Voltage Range V
CC
MAX5025/MAX5027 4.5 11
V
Undervoltage Lockout
Rise/fall, hysteresis = 3mV
V
Supply Current I
CC
MAX5025/MAX5026, FB = 1.4V
MAX5027/MAX5028, FB = 35V
µA
Shutdown Current I
SHDN
SHDN = GND
1µA
BOOST CONVERTER
MAX5025/MAX5027
Switching Frequency f
SW
MAX5026/MAX5028, VCC = 3.3V
670
kHz
MAX5025/MAX5027,
I
LOAD
= 2mA, VCC = 4.5V to 11V,
V
OUT
= 30V
Line Regulation
MAX5026/MAX5028,
I
LOAD
= 0.5mA, VCC = 3V to 11V,
V
OUT
= 30V
%/V
MAX5025/MAX5027,
V
CC
= 5V, I
LOAD
= 0 to 4mA,
V
OUT
= 30V
2.0
Load Regulation
MAX5026/MAX5028,
V
CC
= 3.3V, I
LOAD
= 0 to 1mA,
V
OUT
= 30V
1.0
%
Thermal Shutdown
°C
Thermal Shutdown Hysteresis 2 °C
SYMBOL
MIN TYP MAX
V
UVLO
2.25 2.65 2.95
345 500 1000
410 500
350 1000
0.01
0.25
0.25
140
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC= 5V, SHDN = VCC, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.) (Note 1)
Note 1: All devices are 100% production tested at TA= +25°C. All temperature limits are guaranteed by design.
Note 2: Switch current-limit accuracy is typically ±20% and is a function of the input voltage. I
LIM
= (VIN/5) (260mA).
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX5025, VCC = 4.5V to 11V 1.19 1.25 1.31
FB Set Point V
FB Input Bias Current I
Output Voltage
Adjustment Range
LX OUTPUT
LX On-Resistance R
Switch Current Limit I
LX Leakage Current VLX = 40V
LOGIC INPUT: SHDN
Input Low Level V
Input High Level V
Input Bias Current -1 1 µA
FB
FB
ON
LIM
MAX5027, VCC = 4.5V to 11V 28.5 30.0 31.5
MAX5026, VCC = 3.3V to 11V 1.212 1.25 1.288
MAX5028, VCC = 3.3V to 11V 29.0 30 31
MAX5025/MAX5026, FB = 1V 110 310 nA
MAX5027/MAX5028, FB = 30V 100 170 µA
MAX5025/MAX5026 V
MAX5026/MAX5028,
V
= 3V
ILX = 40mA
Note 2 260 mA
IL
IH
CC
VCC = 5V 1.3 3.0
V
= 11V 1.0 2.5
CC
MAX5025/MAX5026,
V
= 1.4V
FB
MAX5027/MAX5028,
V
= 35V
FB
+ 1 36 V
CC
2.0 4.0
0.01 10 µA
2.4 V
0.8 V
V
Ω
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC= 5V, V
OUT
= 30V, TA= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
= 12V)
(V
90
80
70
60
50
40
EFFICIENCY (%)
30
20
10
0
0246810
OUT
CIRCUIT OF FIGURE 2.
R1 = 147kΩ, R2 = 16.2kΩ
LOAD CURRENT (mA)
MAX5026
V
CC
V
OUT
= 5V,
= 12V
MAX5025-28 toc01
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
= 30V)
(V
70
60
50
40
30
EFFICIENCY (%)
20
10
0
01234
OUT
CIRCUIT OF FIGURE 2.
R1 = 147kΩ, R2 = 6.34kΩ
LOAD CURRENT (mA)
MAX5026
V
CC
= 30V
V
OUT
= 5V,
MAX5025-28 toc04
STARTUP VOLTAGE (V)
MAX5026
NO LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE
16
CIRCUIT OF FIGURE 2
14
12
10
NO LOAD SUPPLY CURRENT (mA)
= 30V
V
OUT
8
6
4
2
0
357911
SUPPLY VOLTAGE (V)
MAX5025-28 toc07
SUPPLY CURRENT (µA)
EFFICIENCY vs. LOAD CURRENT
= 15V)
(V
90
80
70
60
50
40
30
20
10
0
04268
OUT
CIRCUIT OF FIGURE 2.
R1 = 147kΩ, R2 = 13kΩ
LOAD CURRENT (mA)
MAX5026
V
CC
= 12V
V
OUT
MAX5025-28 toc02
EFFICIENCY (%)
= 5V,
MAX5026 MINIMUM STARTUP VOLTAGE
vs. LOAD CURRENT
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
01234
LOAD CURRENT (mA)
MAX5025-28 toc05
SUPPLY CURRENT (µA)
SUPPLY CURRENT vs. TEMPERATURE
850
750
650
550
450
350
250
150
-40 0 20-20 406080
VCC = 11V
VCC = 9V
VCC = 7V
VCC = 5V
VCC = 3V
CURRENT INTO VCC PIN
TEMPERATURE (°C)
MAX5025-28 toc08
SWITCHING FREQUENCY (kHz)
EFFICIENCY vs. LOAD CURRENT
= 24V)
(V
80
70
60
50
40
30
20
10
0
012345
OUT
CIRCUIT OF FIGURE 2.
R1 = 147kΩ, R2 = 8.07kΩ
LOAD CURRENT (mA)
MAX5026
V
CC
V
OUT
= 5V,
= 24V
MAX5026/MAX5028
SUPPLY CURRENT vs. SUPPLY VOLTAGE
500
450
400
350
300
250
200
150
100
50
0
036912
CURRENT INTO V
DEVICE NOT SWITCHING
SUPPLY VOLTAGE (V)
PIN
CC
MAX5026
SWITCHING FREQUENCY vs. TEMPERATURE
650
600
550
500
450
400
350
300
250
200
-40 0 20-20 40 60 80
CIRCUIT OF FIGURE 2.
TEMPERATURE (°C)
VCC = 5V
V
OUT
= 30V
MAX5025-28 toc03
MAX5025-28 toc06
MAX5025-28 toc09
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(VCC= 5V, V
OUT
= 30V, TA= +25°C, unless otherwise noted.)
OUTPUT
VOLTAGE
20V/div
INDUCTOR
CURRENT
50mA/div
SHUTDOWN
VOLTAGE
5V/div
EXITING SHUTDOWN
5V
2ms/div
= 5V, V
MAX5026, V
CIRCUIT OF FIGURE 3
CC
OUT
= 30V, I
LIGHT-LOAD SWITCHING WAVEFORM
WITHOUT RC FILTER
MAX5025-28 toc10
LOAD
MAX5025-28 toc12
= 1mA.
30V
5V
30V
OUTPUT
VOLTAGE
20V/div
SHUTDOWN
VOLTAGE
5V/div
ENTERING SHUTDOWN
MAX5026, V
CIRCUIT OF FIGURE 3
CC
= 5V, V
100ms/div
OUT
= 30V, I
MAX5025-28 toc11
LOAD
= 1mA.
LIGHT-LOAD SWITCHING WAVEFORM
WITH RC FILTER
MAX5025-28 toc13
5V
5V
0V
V
OUT
2mV/div
AC-COUPLED
LX PIN
20V/div
100mA/div
V
OUT
5mV/div
AC-COUPLED
LX PIN
20V/div
200mA/div
I
L
1µs/div
MAX5026, V
CIRCUIT OF FIGURE 2
CC
= 5V, V
OUT
= 30V, I
LOAD
= 0.1mA.
MEDIUM-LOAD SWITCHING WAVEFORM
WITHOUT RC FILTER
I
L
MAX5025-28 toc14
0V
0mA
0V
0mA
V
1mV/div
AC-COUPLED
LX PIN
20V/div
100mA/div
V
OUT
1mV/div
AC-COUPLED
LX PIN
20V/div
200mA/div
OUT
I
L
1µs/div
= 5V, V
MAX5026, V
CC
CIRCUIT OF FIGURE 3
OUT
= 30V, I
LOAD
MEDIUM-LOAD SWITCHING WAVEFORM
WITH RC FILTER
I
L
MAX5025-28 toc15
0V
0mA
= 0.1mA.
0V
0mA
MAX5026, V
CIRCUIT OF FIGURE 2
CC
= 5V, V
1µs/div
OUT
= 30V, I
LOAD
= 2mA.
MAX5026, V
CIRCUIT OF FIGURE 3
CC
= 5V, V
1µs/div
OUT
= 30V, I
LOAD
= 2mA.
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VCC= 5V, V
OUT
= 30V, TA= +25°C, unless otherwise noted.)
MAX5025-28 toc17
LX PIN
20V/div
V
OUT
1mV/div
AC-COUPLED
I
L
200mA/div
0V
0mA
1µs/div
MAX5026, V
CC
= 5V, V
OUT
= 30V, I
LOAD
= 4mA.
CIRCUIT OF FIGURE 3
HEAVY-LOAD SWITCHING WAVEFORM
WITH RC FILTER
LOAD TRANSIENT RESPONSE
MAX5025-28 toc18
OUTPUT
VOLTAGE
200mV/div
AC-COUPLED
LOAD
CURRENT
10mA/div
0mV
0mA
1ms/div
MAX5026, V
CC
= 5V, V
OUT
= 30V, I
LOAD
= 0 TO 4mA.
CIRCUIT OF FIGURE 2
LINE TRANSIENT RESPONSE
MAX5025-28 toc19
OUTPUT
VOLTAGE
1mV/div
AC-COUPLED
INPUT
VOLTAGE
100mV/div
AC-COUPLED
0mV
0mV
2ms/div
MAX5026, V
CC
= 5V TO 5.2V, V
OUT
= 30V, I
LOAD
= 1mA.
CIRCUIT OF FIGURE 2
MAX5025-28 toc20
1.230
1.235
1.245
1.240
1.260
1.265
1.255
1.250
1.270
FB PIN VOLTAGE (V)
-40 0 20-20
40
60
80
TEMPERATURE (°C)
MAX5026
FB PIN VOLTAGE vs. TEMPERATURE
VCC = 11V
VCC = 5V
VCC = 3V
29.0
29.4
29.2
29.8
29.6
30.2
30.0
30.4
-40 0 20-20 40 60 80
MAX5025-28 toc21
TEMPERATURE (°C)
FB PIN VOLTAGE (V)
MAX5028
FB PIN VOLTAGE vs. TEMPERATURE
VCC = 11V
VCC = 5V
VCC = 3V
MAX5025-28 toc16
LX PIN
20V/div
V
OUT
5mV/div
AC-COUPLED
I
L
200mA/div
0V
0mA
1µs/div
MAX5026, V
CC
= 5V, V
OUT
= 30V, I
LOAD
= 4mA.
CIRCUIT OF FIGURE 2
HEAVY-LOAD SWITCHING WAVEFORM
WITHOUT RC FILTER
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
_______________________________________________________________________________________ 7
Typical Operating Characteristics (continued)
(VCC= 5V, V
OUT
= 30V, TA= +25°C, unless otherwise noted.)
0.5
1.0
2.0
1.5
2.5
3.0
-40 0-20 20 40 60 80
MAX5025-28 toc22
TEMPERATURE (°C)
R
ON
(Ω)
SWITCH ON-RESISTANCE
vs. TEMPERATURE
VCC = 3V
VCC = 5V
VCC = 11V
MAX5026
LOAD REGULATION
32
31
30
OUTPUT VOLTAGE (V)
29
28
021 345
WITHOUT RC FILTER
(CIRCUIT OF FIGURE 2)
WITH RC FILTER
(CIRCUIT OF FIGURE 3)
LOAD CURRENT (mA)
MAX5026
= 5V
V
CC
MAX5025-28 toc24
LX LEAKAGE CURRENT vs. TEMPERATURE
0.050
CURRENT INTO LX PIN
LX LEAKAGE CURRENT (µA)
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
= 40V
V
LX
0
-40-200 20406080
TEMPERATURE (°C)
MAX5025-28 toc23
MAX5026
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
100
A
10
1
MAXIMUM LOAD CURRENT (mA)
0.1
311
B
C
D
E
INPUT VOLTAGE (V)
A: V
B: V
C: V
D: V
E: V
OUT
OUT
OUT
OUT
OUT
975
= 12V
= 24V
= 30V
= 32V
= 36V
MAX5025-28 toc25
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
8 _______________________________________________________________________________________
Pin Description
Detailed Description
The MAX5025–MAX5028 current-mode PWM controllers operate in a wide range of DC-DC conversion
applications including boost, flyback, and isolated output configurations. These converters provide lownoise, high output voltages making them ideal for varactor diode tuning applications as well as TFT LCD
bias. Other features include shutdown, fixed 500kHz
PWM oscillator, and a wide input range: 3V to 11V for
MAX5026/MAX5028 and 4.5V to 11V for MAX5025/
MAX5027.
The MAX5025–MAX5028 operate in discontinuous
mode in order to reduce the switching noise at the output. Other continuous mode boost converters generate
a large voltage spike at the output when the LX switch
turns on because there is a conduction path between
the output, diode, and switch to ground during the time
needed for the diode to turn off.
To reduce the output noise even further, the LX switch
turns off by taking 40ns typically to transition from “ON”
to “OFF.” As a consequence, the positive slew rate of
the LX node is reduced and the current from the inductor does not “force” the output voltage as hard as
would be the case if the LX switch were to turn off more
quickly.
PWM Controller
The heart of the MAX5025–MAX5028 current-mode
PWM controllers is a BiCMOS multi-input comparator
that simultaneously processes the output-error signal
and switch current signal. The main PWM comparator
is direct summing, lacking a traditional error amplifier
and its associated phase shift. The direct summing
configuration approaches ideal cycle-by-cycle control
over the output voltage since there is no conventional
error amp in the feedback path.
The device operates in PWM mode using a fixed-frequency, current-mode operation. The current-mode
feedback loop regulates peak inductor current as a
function of the output error signal.
SHDN
Input
The SHDN pin provides shutdown control. Connect
SHDN to VCCfor normal operation. To disable the
device, connect SHDN to GND.
Design Procedure
The MAX5025–MAX5028 can operate in a number of
DC-DC converter configurations including step-up, single-ended primary inductance converter (SEPIC), and
flyback. The following design discussions are limited to
step-up, with a complete circuit shown in the
Application Circuits section.
Setting the Output Voltage
The output voltage of the MAX5027/MAX5028 is fixed
at 30V. The output voltage of the MAX5025/MAX5026 is
set by two external resistors (R1 and R2, Figure 2 and
Figure 3). First select the value of R2 in the 5kΩ to
50kΩ range. R1 is then given by:
PIN
MAX5025/
MAX5026
1 1 PGND
2 2 GND Ground. Connect directly to local ground plane.
3 — FB
— 3FB
44SHDN Shutdown Pin. Connect to VCC to enable device. Connect to GND to shut down.
55VCCInput Supply Voltage. Bypass with a 4.7µF ceramic capacitor.
66LX
MAX5027/
MAX5028
NAME FUNCTION
Power Ground. Connect directly to local ground plane. Use a star ground configuration
for low noise.
Feedback Pin. Reference voltage is approximately 1.25V. Connect resistive-divider tap
here. Minimize trace area at FB. See Setting the Output Voltage section.
Feedback Pin. Connect V
voltage.
Drain of Internal 40V N-Channel DMOS. Connect inductor/diode to LX. Minimize trace
area at this pin to keep EMI down.
to FB for +30V. Internal resistors divide down the output
OUT
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
_______________________________________________________________________________________ 9
where V
REF
is 1.25V
Determining Peak Inductor Current
If the boost converter remains in the discontinuous
mode of operation, then the approximate peak inductor
current, I
LPEAK
, is represented by the formula below:
where T
S
is the period, V
OUT
is the output voltage, V
IN
is the input voltage, I
OUT
is the output current, and η is
the efficiency of the boost converter.
Determining the Inductor Value
47µH is the recommended inductor value when the output voltage is 30V and the input voltage is 5V. In general, the inductor should have a current rating greater
than the current-limit value. For example, the inductor’s
current rating should be greater than 150mA to support
a 4mA output current. Equivalent series resistance
(ESR) should be below 1Ω for reasonable efficiency.
Due to the MAX5025–MAX5028’s high switching frequency, inductors with a ferrite core or equivalent are
recommended. Powdered iron cores are not recommended due to their high losses at frequencies over
500kHz. Table 1 shows a list of vendors and 47µH
inductor parts.
For 4mA output current and output voltages other than
30V, the inductor can be simply scaled in value
according to the following formula:
Use the following formula to calculate the upper bound
of the inductor value at different output voltages and
output currents. This is the maximum inductance value
for discontinuous mode operation.
Calculate the lower bound, L
LOWER
, for the acceptable
inductance value using the following formula, which will
allow the maximum output current to be delivered without reaching the peak current limit:
Notice that the switch current limit, (VIN/5)(260mA), is a
function of the input voltage, VIN. The current rating of
the inductor should be greater than the switch current
limit.
Table 1. Inductor Vendors
Figure 1. Functional Diagram
SHUTDOWN
MAX5025
MAX5026
THERMAL
N
SHDN
PGND
4
6LX
1
MAIN PWM
3
FB
REF
2
GND
5
V
CC
COMPARATOR
-A
+A
+C
-C
RR
CURRENTLIMIT
COMPARATOR
12 1=
OSCILLATORULVO
CONTROL
LOGIC
V
OUT
-
V
REF
I
LPEAK
=
TV VI
2
S OUT IN OUT
−
()
L
η
4725µ
HV V
()
L
=
−
()
OUT IN
V
()
2
VV VT
()
OUT IN S
L
UPPER
IN
=
2
IV
OUT OUT
−
η
2
L
LOWER
TV VI
2
S OUT IN OUT
=
η
−
()
V
IN
260
()
5
mA
2
VENDOR PHONE FAX PART NUMBER OF 47µH INDUCTOR
Coilcraft 847-639-6400 847-639-1469 DT1608C-473
Sumida 847-545-6700 847-545-6720 CDRH4D28-470
Toko 847-297-0070 847-699-7864 A915BY-470M
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
10 ______________________________________________________________________________________
For a design in which VIN= 5V, V
OUT
= 30V, I
OUT
=
4mA, η = 0.5, and TS= 2µs,
L
UPPER
= 87µH
and
L
LOWER
= 12µH.
For a worst-case scenario in which V
IN
= 4.75V, V
OUT
= 29V, I
OUT
= 4.4mA, η = 0.5, and TS= 1.25µs,
L
UPPER
= 46µH
and
L
LOWER
= 9µH.
The choice of 47µH as the recommended inductance
value is reasonable given the worst-case scenario
above. In general, the higher the inductance, the lower
the switching noise. Load regulation is also better with
higher inductance.
Diode Selection
The MAX5025–MAX5028’s high switching frequency
demands a high-speed rectifier. Schottky diodes are
recommended for most applications because of their
fast recovery time and low forward-voltage drop.
Ensure that the diode’s peak current rating is greater
than or equal to the peak inductor current. Also, the
diode reverse breakdown voltage must be greater than
V
OUT
. Table 2 lists diode vendors.
Capacitor Selection
Output Filter Capacitor
The output filter capacitor should be 1µF or greater. To
achieve low output ripple, a capacitor with low ESR, low
ESL, and high capacitance value should be selected.
For very low output ripple applications, the output of the
boost converter can be followed by an RC filter to further reduce the ripple. Figure 3 shows a 100Ω, 1µF filter used to reduce the switching output ripple to
1mVp-p.
X7R ceramic capacitors are better for this boost application because of their low ESR and tighter tolerance
over temperature than the Y5V ceramic capacitors.
Table 3 below lists manufacturers of recommended
capacitors.
Input Capacitor
Bypass VCCwith a 4.7µF ceramic capacitor as close to
the IC as is practical.
Applications Information
Layout Considerations
The MAX5025–MAX5028 switch at high speed, mandating careful attention to layout for optimum performance. Protect sensitive analog grounds by using a
star ground configuration. Minimize ground noise by
connecting GND, PGND, the input bypass-capacitor
ground lead, and the output-filter ground lead to a single point (star ground configuration). Also, minimize
Table 2. Schottky Diode Vendors
Table 3. Capacitor Table
VENDOR PHONE FAX PART NUMBERS
Comchip 510-657-8671 510-657-8921 CDBS1045
Panasonic 408-942-2912 408-946-9063 MA2Z785
ST-Microelectronics 602-485-6100 602-486-6102 TMMBAT48
Vishay-Telefunken 402-563-6866 402-563-6296 BAS382
Zetex 631-360-2222 631-360-8222 ZHCS500
COMPANY PHONE FAX PART NUMBERS
Murata 814-237-1431 814-238-0490
Taiyo Yuden 408-573-4150 408-573-4159
TDK 847-803-6100 847-803-6296
GRM42-2X7R105K050AD (1µF capacitor)
GRM32-1210R71C475R (4.7µF capacitor)
UMK325BJ105KH (1µF capacitor)
EMK316BJ475ML (4.7µF capacitor)
C3225X7R1H155K (1.5µF capacitor)
C3225X7R1H105K (1µF capacitor)
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
______________________________________________________________________________________ 11
trace lengths to reduce stray capacitance, trace resistance, and radiated noise. The trace between the output voltage-divider (MAX5025/MAX5026) and the FB
pin must be kept short, as well as the trace between
GND and PGND.
Inductor Layout
The shielded drum type inductors have a small air gap
around the top and bottom periphery. The incident fringing magnetic field from this air gap to the copper plane
on the PC board tends to reduce efficiency. This is a
result of the induced eddy currents on the copper plane.
To minimize this effect, avoid laying out any copper
planes under the mounting area of these inductors.
30V Boost Application Circuit
Figures 2 and 3 show the MAX5025/MAX5026 operating in a 30V boost application. Figure 3 has an RC filter
to reduce noise at the output. These circuits provide
output currents greater than 4mA with an input voltage
of 5V or greater. They are designed by following the
Design Procedure section. Operating characteristics of
these circuits are shown in the Typical Operating
Characteristics section.
____________________Chip Information
TRANSISTOR COUNT: 365
PROCESS: BiCMOS
Figure 2. Adjustable 30V Output Circuit
Figure 3. Adjustable 30V Output Circuit with RC Filter
VCC = 4.5V TO 11V
(MAX5025)
= 3V TO 11V
V
CC
(MAX5026)
C1
4.7µF
V
CC
SHDN
PGND
TOKO 47µH INDUCTOR
L1
A915BY-470M
47µH
LX
MAX5025
MAX5026
FB
GND
ZETEX SCHOTTKY DIODE
ZHCS500
D1
R1
147kΩ
R2
6.34kΩ
C2
C2
1µF
1µF
V
OUT
+30V
VCC = 4.5V TO 11V
(MAX5025)
= 3V TO 11V
V
CC
(MAX5026)
C1
4.7µF
V
CC
SHDN
PGND
TOKO 47µH INDUCTOR
L1
A915BY-470M
47µH
MAX5025
MAX5026
GND
C2
1µF
R3
100Ω
C3
1µF
V
OUT
+30V
ZETEX SCHOTTKY DIODE
ZHCS500
LX
FB
D1
R1
147kΩ
R2
6.34kΩ
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
12 ______________________________________________________________________________________
Selector Guide
PART OUTPUT
MAX5025 Adjustable -31% to +100% ±5% 4.5V to 11V
MAX5026 Adjustable -18% to +34% ±3% 3V to 11V
MAX5027 Fixed 30V -31% to +100% ±5% 4.5V to 11V
MAX5028 Fixed 30V -18% to +34% ±3% 3V to 11V
FREQUENCY
TOLERANCE
FB SET POINT
TOLERANCE
INPUT VOLTAGE
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13
© 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
MAX5025–MAX5028
500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
Package Information
6LSOT.EPS
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