19-0766; Rev 1; 3/08
MAX8790A Evaluation Kit
General Description
The MAX8790A evaluation kit (EV kit) is a fully assem-
bled and tested surface-mount PCB that evaluates the
high-efficiency MAX8790A white LED (WLED) driver.
The MAX8790A EV kit utilizes a step-up DC-DC con-
verter to generate the voltage required to drive up to six
strings of eight surface-mount WLEDs. The EV kit is
powered by a 7V to 21V input power supply (single-
supply mode) and can be configured to provide a fixed
20mA or adjustable 15mA to 25mA full-scale LED cur-
rent. In addition, the MAX8790A provides LED current
control through either analog or digitally adjusted
pulse-width modulation (DPWM) dimming modes. As
shipped, the MAX8790A EV kit is configured for evalua-
tion with two strings of eight WLEDs.
♦
7V to 21V Input Range
♦
86% Efficiency (VIN = 12V, Load = Two WLED
Strings)
♦
WLED Drives Up to 25mA/String
♦
Drives Two Strings of Eight WLEDs (Capable of
Driving Up to Six Strings)
♦
Analog or Direct DPWM Dimming Control
♦
100:1 Dimming Range
♦
750kHz PWM Switching Frequency (Selectable:
1MHz or 500kHz, Component Change Required)
♦
Fully Assembled and Tested
Features
Evaluates: MAX8790A
Ordering Information
PART
MAX8790AEVKIT+
TYPE
EV Kit
+Denotes
lead-free and RoHS-compliant.
Component List
DESIGNATION QTY
DESCRIPTION
0.1µF ±10%, 50V X7R ceramic
capacitors (0603)
Murata GRM188R71H104K
TDK C1608X7R1H104K
1µF ±10%, 6.3V X5R ceramic
capacitor (0603)
Murata GRM188R60J105K
TDK C1608X5R1A105K
2.2µF ±10%, 50V X7R ceramic
capacitor (1206)
Murata GRM31CR71H225K
KEMET C1206C225K5RAC
Not installed, ceramic capacitors
(1206)
10µF ±10%, 25V X5R ceramic
capacitor (1206)
Murata GRM31CR61E106KA
TDK C3216X5R1E106K
Not installed, ceramic capacitors
(0603)
DESIGNATION QTY
D1
1
DESCRIPTION
2A, 40V Schottky diode (M-Flat)
Toshiba CMS11 (TE12L, Q)
Nihon EC21QS04
White LEDs
Nichia NSSW008C
OPTEK OVSRWACR6
Not installed, white LEDs
3-pin headers
2-pin headers
Not installed, 3-pin headers
4.7µH, 2.15A, 64.1mΩ power inductor
TDK LTF5022T-4R7N2R0
Sumida CDRH5D16-4R7 (1.8mm)
60V, 2.8A n-channel MOSFET
(6 TSOP)
SANYO Semiconductor CPH6424
(Top Mark: ZA)
Fairchild Semiconductor FDC5612
(Top Mark: 562)
C1, C3, C4
3
D2–D17
D18–D49
JU1, JU3,
JU6, JU7
JU2, JU4,
JU5, JU12
JU8–JU11,
JU13
L1
16
0
4
4
0
C2
1
C5
1
C6, C8,
C11–C16
0
1
C7
1
C9, C10, C17
0
N1
1
________________________________________________________________
Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
MAX8790A Evaluation Kit
Evaluates: MAX8790A
Component List (continued)
DESIGNATION QTY
R1
R2
R3
R4
R5
R6
1
1
1
1
1
1
DESCRIPTION
51kΩ ±5% resistor (0603)
100kΩ potentiometer, multiturn
511kΩ ±1% resistor (0603)
37.4kΩ ±1% resistor (0603)
1MΩ ±1% resistor (0603)
56mΩ ±1%, 0.5W resistor (1206)
IRC LRC-LRF-1206LF-01-R056-F
Vishay WSL1206R0560FEA18
Not installed, resistor—short
(PC trace) (0603)
U1
—
—
1
8
1
DESIGNATION QTY
R8
R9, R10
R11–R16
1
2
0
DESCRIPTION
1.2kΩ ±5% resistor (0603)
100kΩ ±5% resistors (0603)
Not installed, resistors (0603)
Six-string white LED driver
(20 TQFN)
Maxim MAX8790AETP+
Shunts
PCB: MAX8790A Evaluation Kit+
R7
0
Component Suppliers
SUPPLIER
Fairchild Semiconductor
IRC, Inc.
KEMET Corp.
Murata Electronics North America, Inc.
Nichia Corp.
Nihon Inter Electronics Corp.
OPTEK Technologies
SANYO Electric Co., Ltd.
Sumida Corp.
TDK Corp.
Toshiba America Electronic Components, Inc.
PHONE
888-522-5372
361-992-7900
864-936-6300
770-436-1300
248-352-6575
847-843-7500
972-323-2200
619-661-6835
847-545-6700
847-803-6100
949-623-2900
www.irctt.com
www.kemet.com
www.murata-northamerica.com
www.nichia.com
www.niec.co.jp
www.optekinc.com
www.sanyodevice.com
www.sumida.com
www.component.tdk.com
www.toshiba.com/taec
WEBSITE
www.fairchildsemi.com
Vishay
402-564-3131
www.vishay.com
Note:
Indicate that you are using the MAX8790A when contacting these component suppliers.
Quick Start
Recommended Equipment
Before beginning, the following equipment is needed:
• 25V, 1A power supply (VIN)
2) Connect the positive terminal of the VIN power sup-
ply to the VIN pad on the EV kit and connect the
ground terminal of the VIN power supply to the
PGND pad on the EV kit.
3) Set the VIN power supply to 12V and enable its
output.
4) Verify that both WLED strings are lit.
Procedure
The MAX8790A EV kit is fully assembled and tested.
Follow the steps below to verify board operation.
Caution: Do not turn on the power supply until all
connections are completed.
1) Verify that the shunts are installed as follows:
JUMPER
JU1, JU6, JU7
JU2, JU4
JU3
JU5, JU12
DEFAULT SHUNT POSITION
1-2
Not installed
2-3
Installed
Detailed Description
The MAX8790A EV kit is a high-efficiency WLED driver
that utilizes a step-up DC-DC converter optimized to gen-
erate the necessary output voltage for driving six strings
of WLEDs. The EV kit is powered by a 7V to 21V input
power supply and can be configured to provide a fixed
20mA or adjustable 15mA to 25mA full-scale LED current.
2
_______________________________________________________________________________________
MAX8790A Evaluation Kit
The EV kit is populated with two strings of eight WLEDs,
but provides four additional on-board strings to allow
for the evaluation of up to six total strings of eight
WLEDs (48 total WLEDs). In addition, the EV kit can be
used to evaluate external off-board WLED strings, pro-
viding flexibility in the layout of the WLED strings and
allowing for the evaluation of more WLEDs per string.
See the
White LED String Configuration
section for
WLED configuration options.
LED current control is accomplished through either
analog or digital dimming modes, which achieve a
100:1 dimming range. Both dimming modes utilize the
pulse-width modulation (PWM) signal supplied at the
BRT input pad to control the LED current sources. See
the
LED Dimming Mode
section for more details.
As configured, the MAX8790A EV kit operates with a
750kHz switching frequency and an output voltage of
35V. Operating the EV kit at a different switching fre-
quency or output voltage might require component
changes. Refer to the MAX8790A IC data sheet for
details on component selections.
Switching-Frequency Selection (OSC)
The MAX8790A EV kit is configured to operate at
750kHz, but provides the option to set the switching fre-
quency of the step-up DC-DC converter to 500kHz or
1MHz. The switching frequency is set by installing head-
er JU13 and configuring the shunt location as listed in
Table 3. Changing the switching frequency may require
different converter components. Refer to the MAX8790A
IC data sheet for proper component selections.
Evaluates: MAX8790A
Setting Full-Scale LED Current (I
LED(FS)
)
The full-scale current through each WLED string is con-
figured by connecting ISET (pin 18) to VCC, or by tying
it to GND through resistors R1 and R2. When ISET is
connected to VCC the full-scale current is set to 20mA,
and when pulled to GND the full-scale current is
adjustable from 15mA to 25mA:
⎛
100k
Ω ⎞
I
LED(FS)
= ⎜
20 x
mA
R1+ R2
⎟
⎝
⎠
where R1 is a 51kΩ resistor and R2 is a 100kΩ poten-
tiometer. The ISET pin is configured through jumper
JU4. See Table 4 for jumper JU4 settings.
Input Power Supply
The MAX8790A EV kit can be evaluated with either one
or two input power supplies. When using one input
power supply, connect a 7V to 21V power supply to the
VIN pad and configure jumper JU1 (Table 1) for single-
supply operation. In single-supply operation, do not con-
nect an external power supply or load to the VCC pad.
To utilize two input power supplies, configure jumper
JU1 for dual-supply operation and connect a 4.5V to
5.5V power supply to the VCC pad and a 7V to 21V
power supply to the VIN pad.
Table 2. Jumper JU2 Function
SHUNT
POSITION
Not installed*
Installed
SHDN
PIN
Connected to VIN
through R9
Connected to GND
MAX8790A
Enabled
Disabled
Shutdown Mode (
SHDN
)
The MAX8790A is disabled by connecting
SHDN
to
GND. When in shutdown mode, the IC quiescent cur-
rent is reduced to less than 10µA (typ). The
SHDN
input
is set through jumper JU2. See Table 2 for shutdown
configurations.
*Default
position.
Table 3. Jumper JU13 Function
SHUNT
POSITION
1-2
2-3
Not installed*
OSC PIN
Connected to VCC
Connected to GND
Unconnected
OPERATING
FREQUENCY
1MHz
500kHz
750kHz
Table 1. Jumper JU1 Function
SHUNT POSITION
1-2*
2-3
INPUT SUPPLY
Single (7V to 21V)
Dual (VIN = 7V to 21V, VCC = 4.5V
to 5.5V)
*Default
position.
*Default
position.
_______________________________________________________________________________________
3
MAX8790A Evaluation Kit
Evaluates: MAX8790A
Table 4. Jumper JU4 Function
SHUNT
POSITION
Installed
Not installed*
ISET PIN
Connected to VCC
Connected to GND
through R1 and R2
ILED(FS)
20mA
Adjustable from
15mA to 25mA
*Default
position.
LED Dimming Mode
The MAX8790A provides a 100:1 dimming range
through analog or digital control methods. In both meth-
ods, the BRT input signal is translated into a control sig-
nal for each of the six current sources. By varying the
BRT duty cycle, the LED string current (I
LED
) is adjust-
ed between 0mA and the full-scale current setting. See
the
Setting Full-Scale LED Current (I
LED(FS)
)
section for
details on configuring the full-scale current. To select
the dimming mode, configure jumper JU3 as shown in
Table 5, and see the
Direct DPWM Dimming Mode
and
Analog + DPWM Dimming Mode
sections for additional
information.
Analog + DPWM Dimming Mode
To enable analog + DPWM dimming, configure jumper
JU3 as listed in Table 5, and ensure that a shunt is not
installed across jumper JU5.
In analog dimming mode, I
LED
is a DC signal whose
amplitude is modulated by the duty cycle of the BRT
input signal. Therefore, at 50% BRT duty cycle, the I
LED
DC signal is set to 50% of the full-scale current. While in
analog dimming mode, the BRT frequency is limited to
the capture range of the internal PLL. The free-running
frequency of the PLL is set by R3 to 244Hz, providing a
capture range of 147Hz to 244Hz. Refer to the
MAX8790A IC data sheet for information on configuring
the PLL’s free-running frequency.
This analog control scheme holds true down to a BRT
duty cycle of 12.5%. Below 12.5%, the string currents
are modulated through a combination of analog and
digital dimming schemes. Refer to the MAX8790A IC
data sheet for further details.
White LED String Configuration
As configured, the MAX8790A EV kit is assembled with
two strings of eight WLEDs (String1 and String2), but
can be reconfigured to drive up to six strings of eight
WLEDs. Each string has an associated 3-pin header
(JU6–JU11) and feedback pin (FB1–FB6). To evaluate
additional strings (String3–String6), the EV kit must be
reconfigured as follows:
1) Populate the String_ with either the WLED listed in
the
Component List
or its equivalent.
2) Cut the trace (solder side) between pins 2-3 of the
string’s associated header-footprint (JU8–JU11).
3) Install a 3-pin header (JU8–JU11) and configure its
shunt according to Table 6.
To evaluate the EV kit with off-board WLED strings, see
the
Off-Board White LED String Configuration
section.
Direct DPWM Dimming Mode
To enable DPWM dimming, configure jumper JU3 as
listed in Table 5, and ensure that a shunt is installed
across jumper JU5.
In direct DPWM dimming mode, I
LED
is a PWM signal
whose amplitude is equal to the full-scale current set-
ting. The duty cycle of the I
LED
signal is synchronized
with the BRT signal, such that at 50% BRT duty cycle
the I
LED
duty cycle are also 50%. While in direct DPWM
dimming mode, the internal phase-locked loop (PLL) is
not used and does not limit the BRT frequency. Refer to
the MAX8790A IC data sheet for information on deter-
mining the maximum allowable BRT frequency.
Table 5. Jumpers JU3, JU5 Function
JU3
1-2
2-3*
ENA PIN
Connected to VCC
Connected to GND
JU5 (FSET)
Not installed (connected to GND through R3)
Installed (connected to VCC)
DIMMING MODE
Analog + DPWM
Direct DPWM
*Default
position.
4
_______________________________________________________________________________________
MAX8790A Evaluation Kit
Evaluates: MAX8790A
Table 6. Jumpers JU6–JU11 Function
SHUNT
POSITION
1-2
2-3
Not installed
FB_ PIN
Connected to cathode
of WLED string
Connected to GND
Connect to an external
WLED string
STRING_
Enabled
Disabled
On-board WLED
string not used
Table 7. Jumper JU12 Function
SHUNT POSITION
Installed*
Not installed
VOUT
Connected to anodes of on-board
WLED strings
Connect to anodes of off-board
WLED strings
*Default
position.
Off-Board White LED String Configuration
The MAX8790A EV kit is also used to drive off-board
WLED strings. This allows the evaluation of up to 10
WLEDs per string. To evaluate external WLED strings,
remove the shunts from jumpers JU6–JU12. See Table
6 for jumpers JU6–JU11 and Table 7 for jumper JU12
configurations. Removing these jumpers effectively dis-
connects the on-board WLED strings from between the
output and feedback pins, allowing the connection of
external WLED strings. For each external WLED string,
connect the cathode terminal of the string to the corre-
sponding feedback pad (FB1–FB6), and connect the
anode terminal of the string to the VOUT pad. Once the
external WLED strings are connected between the
VOUT pad and the FB_ pins, the EV kit can be evaluat-
ed in the same manner as with the on-board WLED
strings. Evaluating more than eight WLEDs per string
could require component changes. Refer to the
MAX8790A IC data sheet for component selections.
Note:
When evaluating more than eight WLEDs per
string, ensure that at low currents the forward voltage
drop across the strings is large enough to avoid
exceeding the 28V maximum rating of the FB pins [V
FB
= (V
OUT
- V
F_WLEDs
) < 28V].
LED String Capacitance
In some LCD panel applications, a 0.1µF (typ) capaci-
tor, C
LED
, is placed in parallel with each LED string to
improve ESD immunity. As such, the MAX8790A EV kit
provides a footprint across each LED string in addition
to footprints for pullup resistors on each FB_ pin. Refer
to the
LCD Panel Capacitance
section in the
MAX8790A IC data sheet for more detailed information.
_______________________________________________________________________________________
5
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