Application Notes

AN11470
Leadless Schottky diodes in a DC-to-DC step-up converter
Rev. 1 — 22 April 2014
Application note
Document information
Info
Content
Keywords
Schottky diode, boost converter, LED backlight, current source
Abstract
This application note discusses the application of DC-to-DC voltage up
conversion with focus on the Schottky diode used in the backlight unit of
typical mobile devices. Products equipped with a Liquid Crystal Display
(LCD) such as smartphones, tablet PCs or notebooks need backlight for
the illumination. Strings of LEDs are the standard solution for an energy
efficient light generation. The LED strings need to be supplied with a
defined current. The voltage required for the strings is a multiple of the
forward voltage of the LEDs used. The main supply voltage of mobile
devices, e.g. from a lithium-ion battery pack, is not high enough to run a
current through the LED string. Therefore a voltage booster is required.
Requirements for the Schottky diode in this booster with respect to
electrical and thermal performance are discussed in detail. NXP
Semiconductors Schottky diodes allow to replace diodes in comparably
large packages with very small components and enable a further step
towards miniaturization without compromising performance.
AN11470
NXP Semiconductors
Leadless Schottky diodes in a DC-to-DC step-up converter
Revision history
Rev
Date
Description
1
20140422
Initial version
Contact information
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: salesaddresses@nxp.com
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Leadless Schottky diodes in a DC-to-DC step-up converter
1. Introduction
Most of the mobile communication and computing devices like mobile phones, tablet PCs,
navigation systems and notebooks use LCDs. An LCD display needs backlight that is
placed behind the LCD shutter system. For each pixel of the display LCD crystals can
block light transmission when a black pixel is displayed or they let light pass to generate
e.g. a white pixel. The degree of transmissibility is controlled for each pixel to allow gray
level in between. In modern LCD devices white LEDs are chosen as an energy efficient
light source. They combine advantages like a long life time, a constant white temperature
over time and low heat dissipation. The light produced by LEDs is distributed with plastic
light spreaders to achieve a uniform brightness for all areas of the display. White LEDs
have a forward voltage VF of 3.2 V to 3.6 V. The LEDs are clustered into strings, where a
defined number of LEDs is placed in series to build a string. The forward voltage of a
string is the multiple of VF of a single LED.
The supply voltage of mobile devices is usually lower than the voltage required to drive
LED strings for the LCD backlight. In smartphones normally a single lithium-ion cell
provides roughly 3.8 V. Therefore a voltage up conversion or a boosting of the battery
voltage towards a suitable voltage of the backlight is required. The voltage conversion has
to work like the LEDs themselves with a high energy efficiency. Furthermore small and low
weight components have to be chosen for the circuits.
The voltage up conversion is usually implemented in a switch mode topology. The
conversion block requires an electronic switch, a diode, an inductor and a capacitor as
discrete key components.
This application note focuses on the Schottky diodes that can be found in backlight
step-up converters. New package technologies and enhanced Schottky processes allow
more compact designs in combination with an excellent energy efficiency.
AN11470
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Leadless Schottky diodes in a DC-to-DC step-up converter
2. Methods of DC-to-DC voltage up conversion
2.1 Voltage up conversion based on charge pumps
There are several technical options how to convert a low DC input voltage VI to a higher
output voltage VO. One possibility is to use a transformer where the primary winding is
connected to a chopper circuit. At the secondary side the output voltage needs to be
rectified. Such a transformer approach requires two coupled coils and therefore too much
heavy material. Galvanic decoupling is a feature of this solution, but not required for a
backlight booster in mobile devices.
Another solution for voltage up conversion is a charge pump. Figure 1 depicts a circuit
that can provide an output voltage twice as high as the input voltage in case of ideal
components. With real components the forward voltage drop of the diodes leads to a
lower output voltage. Assuming the switch SW is connected to the ground GND, capacitor
C1 is charged to VI VF via the diode D1, capacitor C2 reaches the voltage VI 2VF via
the diode D2. When the switch is connected to VI now, the charged capacitor C1 gets
connected to this higher reference point. Now charge can flow into C2 via D2 while D1 is
blocking in reverse direction. If the switch is connected to the ground again, C1 is
recharged and D2 is driven in reverse direction, because the voltage at C2 is higher than
VI. After some switching cycles the output voltage equals:
VO = 2   VI – VF 
SW
C1
GND
VI
D1
D2
VO
C2
GND
Fig 1.
AN11470
Application note
R1
GND
Voltage up conversion based on a charge pump
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Leadless Schottky diodes in a DC-to-DC step-up converter
Figure 2 shows an extension of the above described circuit. This structure can be used to
achieve a triplication of the input voltage. The circuit depicted in Figure 1 is extended by
two additional diodes D3 and D4 and two additional capacitors C3 and C4. The function of
the voltage doubler has been explained already. The doubled voltage at C2 gets
transferred via D3 and D4 to C4. Via the switched capacitor C3, an additional voltage of VI
is finally added at C4 after some switching cycles, assuming the ideal case that the diodes
have no forward losses. In practice as diodes have a forward loss, the output voltage
reaches only
VO = 3VI 4VF. The voltage up converter discussed can be extended with further
booster stages. This way charge pump principle can be used to achieve a boosting factor
of n if 2(n 1) diodes and capacitors are arranged according to the structure explained
above. The general formula for VO and a boosting factor n is:
VO = n  VI – 2  VF   n – 1 
A disadvantage of the circuit introduced is that the forward losses of the diodes have a
comparably big impact on the boosting factor, if the input voltage is quite small. Generally
the number of components increases if a bigger boosting factor is required. Furthermore
the structure is suitable for rather small load currents only. Therefore charge pump
solutions can be found in applications such as gate voltage booster in synchronous
voltage down converters, where a gate voltage on top of VI is required for the high-side
switch (see Ref. 1 “Application Note AN11119 Medium power small-signal MOSFETs in
DC-to-DC conversion.”). Another application example of a charge pump is a voltage
inverter used for an operational amplifier where a negative supply voltage shall be
generated from a single positive supply. In Integrated Circuit (IC) designs synchronous
rectification can be found, in which MOSFETs parallel to the Schottky diodes are switched
on at the moment where the diodes are in forward conduction mode. This can reduce the
losses of the charge pump design significantly.
C3
SW
C1
GND
VI
D1
D2
D3
D4
VO
C2
GND
Fig 2.
AN11470
Application note
C4
GND
R1
GND
Charge pump circuit to generate a voltage boosting factor of 3
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Leadless Schottky diodes in a DC-to-DC step-up converter
2.2 Inductor-based voltage up converter
The major part of voltage boosters is based on an inductor, used to store energy in the
process of the voltage up conversion. The standard topology is quite simple as shown on
Figure 3. It consists of an inductor L1 that is connected to a supply voltage at one side and
can be switched to the ground via the low-side MOSFET switch Q1. From the switching
node the diode D1 builds a path to the output capacitor C2. Assuming ideal and lossless
components, the output capacitor gets charged to VI once the input supply is turned on. If
Q1 is switched on, the current through the inductor increases linearly and magnetic
energy is stored. The amount of energy is:
1
2
E = ---  L  I
2
The current increases if losses are neglected by I:
1
I = ---  V I  t on
L
The diode is driven in reverse direction, this means that it blocks a current flow. The anode
is connected to the ground and the cathode voltage is VO. If the switch is turned off, the
current through the inductor continues to flow in the same direction and the diode works in
forward. Charge is transferred into the output capacitor. The inductor current decreases
with a linear curve (I0 A):
I = 1L   V O – V I   t off
The current through the inductor has a triangular waveform. If the current stays larger than
0 A for all the time in the circuit, the operation condition is called continuous mode.
VI
L1
C1
CONTROL
D1
VO
Q1
C2
GND
Fig 3.
AN11470
Application note
Non-synchronous DC-to-DC step-up converter
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Leadless Schottky diodes in a DC-to-DC step-up converter
For I following equations can be used to calculate VO:
(1)
V O = V I   t on + t off   t off
(2)
D = t on   t on + t off  ;T = t on + t off
(3)
V O = V I  T  t off = V I  T   T – t on  = V I   1 – D 
(4)
VO = VI   1 – D 
The simple equation can be understood easily, if the corner cases for the duty cycle D are
checked. If D is zero, this means the switch stays in off condition all the time, VO is equal
VI. In practice the forward loss of the diode needs to be subtracted. If D is increased and
gets close to 1, the output voltage reaches infinite values. In practice however the current
through the inductance has to stay below a certain value, where a saturation of the
magnetic material happens. Maximum drain current for the switching MOSFET indicates
another limit. Furthermore the maximum voltage ratings of the diode, the MOSFET and
the output capacitor needs to be obeyed. The control of the duty cycle is getting more and
more tough if very high boosting factors are foreseen, because the curve of VO versus D
gets very steep if the duty cycle is close to 1. This means that the system reacts
intensively with respect to VO if the switching times are changed by a small step.
In Figure 4 an example of a SPICE circuit diagram created with LTspice software of Linear
Technology Corporation for a DC-to-DC step-up converter is shown. As a switch
PMV16UN, a low RDSon N-channel MOSFET is selected. Drain-source on-state resistance
is typically RDSon = 15 m at a gate-source voltage VGS = 4.5 V only. As Schottky diode a
PMEG3010ER is chosen. This diode has rated reverse voltage of
VR = 30 V and continuous forward current of IF = 1 A. An output capacitor of 100 F and a
load resistor of 25  are applied.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Fig 4.
DC-to-DC step-up converter, SPICE example with Schottky diode PMEG3010ER
and N-channel MOSFET PMV16UN
With supply voltage VI = 4.5 V, the MOSFET is switched by the source Vctrl. This
generates a gate control signal with 4.5 V high state level, 0 V low level and a frequency of
50 kHz. The duty cycle is 0.75. This means that the switch is turned on for 15 s per 20 s
period time. Figure 5 shows the SPICE simulation results for this voltage booster
example. The red curve is the voltage curve of the switching node. While the MOSFET is
switched on, the voltage is close to ground. If the switch is turned off, the voltage jumps to
VO, plus the forward voltage of the diode on top. The inductor current is depicted in green
(curve 3) and shows triangle waveform. While the switch is closed, the current increases
linearly. During off-time of the switch, the current decreases linearly again. The output
voltage is shown with the blue trace (curve 1). According to theory and the output voltage
formula (4), for the ideal case:
V O = 4 5   1 – 0 75  = 4 5  4 = 18V
The simulation delivers 17.82 V as averaged value with the consideration of realistic
components. This means that switching times are taken into account producing switching
losses in the MOSFET. Furthermore the residual on-state resistance of the switch needs
to be considered and the finite capacitance of the output capacitor.
The output current is IO = 0.712 A DC current, if the small ripple is disregarded. The
current through the diode is the part of the current through the inductor for the time the
switch is opened. This is equivalent to the sections of the triangle-shaped waveform
where the current decreases. The average value of the diode current (triangle +
rectangular) needs to be identical to the output current. Therefore the average value for
each current pulse has to be (pulse average): I D1 = I O   1 – D 
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Leadless Schottky diodes in a DC-to-DC step-up converter
If the current is adjusted to a value where the inductor current waveforms just touches the
0 A line, the system operates at the edge towards the continues mode. In this case there
is no rectangular content which needs to be added to the triangle.
For the peak current the equation can be defined:
I D1max = 2  I O   1 – D 
For this kind of operation there is the biggest factor between the output current and the
peak current in the diode. The example shows that the diode in the voltage boosters
works with comparably high currents, present at the time it is in conductive mode. The
diode has to be chosen taking this fact into account. The operating point in the IFVF
diagram is located at much higher currents than the output current itself. Therefore a low
VF characteristic increases power efficiency a lot.
22
V
(V)
(1)
4.8
(3)
I
(A)
18
4
14
3.2
10
2.4
6
1.6
(4)
(2)
2
-2
4.070
0.8
4.076
4.082
4.088
4.094
t (ms)
0
4.100
(1) VO output voltage, blue
(2) VSW switching voltage, red
(3) IL1 inductor current, green
(4) IR1 resistor current, black
Fig 5.
AN11470
Application note
DC-to-DC step-up converter, SPICE simulation with Schottky diode PMEG3010ER
and N-channel MOSFET PMV16UN
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Leadless Schottky diodes in a DC-to-DC step-up converter
3. Demo PCB DC-to-DC step-up converter using very small Schottky
diodes
3.1 Hardware of the DC-to-DC step-up converter on demo PCB
To demonstrate the technical potential of NXP Semiconductors small package Schottky
diodes a demo PCB for an LED booster has been designed. It can be supplied with an
input voltage starting from 1.6 V. The input voltage is boosted based on a voltage up
converter structure as explained in the last chapter. A controller LT1618 from Linear
Technology Corporation is used for the switching task. It can work in constant current and
voltage control mode. LT1618 was chosen as an easy to handle device with a low pin
count. Figure 6 shows the schematic diagram of the demo PCB.
IN
L1
LPS6225
1
2
CON1
D1
10 μH
C1
10 μF
GND GND
GND
SHDN
n.c.
IAdj
CON2
D2
VIN
IC1
SW
8
7
3
9
LT1618
6
2
4
1
5
GND
PMEG2002AESF
ISP
ISN
R2
332 kΩ
FB
10
GND
VC
C3
100 nF
GND
Fig 6.
0.15 Ω
PMEG2005BELD
C2
100 nF
1 OUT
2
3
R1
GND
C4
10 μF
GND
R3
124 kΩ
GND
aaa-011230
Schematic diagram of LED booster demo PCB with Schottky diode
PMEG2005BELD (DFN1006D-2 / SOD882D) or
PMEG2002AESF (DSN0603-2 / SOD962)
The LT1618 works with a constant switching frequency of 1.4 MHz. This allows the usage
of small inductors and output capacitors. The operating voltage range is 1.6 V to 18 V. The
switch in the LT1618 is realized with a bipolar transistor. The controller can support an
output voltage control and an output current control. An inductance of 10 H is selected
for L1 and an output capacitor of 10 F. For the booster diode function two footprints (D1
and D2) are available. Only one of these diodes shall be assembled. One diode is
foreseen in an DFN1006D-2 (SOD882D) package with a size of
1 mm 0.6 mm 0.37 mm, the other assembly option is the smaller
DSN0603-2 (SOD962) package with a size of 0.6 mm  0.3 mm  0.3 mm. R1 is the
current sensing resistor. The current sensing voltage at R1 is limited to 50 mV by the
current control of LT1618. This leads to the following equation for the output current:
I O = 50mV  R1
With R1 = 0.15 like on Figure 6, the output current is set to 333 mA. This is the suitable
current in case a 1 W power LED is foreseen as load for the demo PCB.
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Leadless Schottky diodes in a DC-to-DC step-up converter
At the FeedBack (FB) pin the controller IC monitors the output voltage. Even if the board
is run in a current source application an additional voltage control for the situation of an
open output is an important safety feature. A simple current source would increase the
output voltage if driven without any load or if driven with a more high-ohmic load than
foreseen for the design. The high output voltage could damage the switching transistor
inside the controller IC, exceed the maximum allowed reverse voltage of the Schottky
diode or the voltage rating of the output capacitor C1. LT1618 limits the voltage at FB pin
to 1.263 V. The output of the booster is connected via the resistor divider built by R2 and
R3 to the FB pin. This leads to the following equation for VO:
V O = 1.263V   R2 + R3    R3 
In a current source application the voltage limiter should be set to an output voltage that is
20% to 30% higher than the voltage that a nominal LED load requires. This ensures that
the current control does not interfere with the voltage control in the normal operation mode
with a LED load. The voltage limitation is a good protection against too high voltages if a
wrong load is applied to the booster. The dimensioning of the voltage feedback divider as
defined in the schematic diagram on Figure 6 leads to a maximum output voltage of about
IO = 4.64 V.
Fig 7.
AN11470
Application note
Placement plan of the components on the demo PCB
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Leadless Schottky diodes in a DC-to-DC step-up converter
Figure 7 shows the placements of the components on the demo PCB. The input connector
uses a two-pin plug, whereas the output uses 3 pins. In this way a risk of commutation for
the external connections to the input supply and the LED load is avoided mechanically.
For the set-up of the PCB it is important to take care that VI is never higher than the
programmed VO. Otherwise a high current can flow directly via the Schottky diode D1 and
the LED load due to the steep forward current versus forward voltage characteristics of
both diode devices.
Figure 8 shows simulated traces for the use case where an input voltage VI = 2 V is
applied as input voltage and a 1 W white LED is connected as load. The booster diode
simulated is the PMEG2005BELD. Curve 1 (green) shows the switching node SW
according to Figure 6. Curve 2 (blue) is the output voltage, which is roughly VO = 3.2 V.
Curve 3 (red) depicts the current through the diode. The trace shows that the ripple is
rather small with the 10 H inductor at the fixed switching frequency of 1.4 MHz.
4
V
(V)
1.8
(1)
I
(A)
3
1.4
(2)
2
1
1
0.6
(3)
0
-1
0.000
0.2
0.200
0.400
0.600
0.800
t (μs)
-0.2
1.000
(1) VSW switching node voltage, green
(2)
ID1 diode current, red
(3) VO output voltage, blue
Fig 8.
SPICE simulation of a single 1 W white LED load and VI = 2 V
In the SPICE schematic diagram in Figure 9 the output load has been changed to two
LEDs in series. R3 needs to be adapted to allow a higher output voltage. The input
voltage has been changed to 3 V to represent a supply built with two batteries.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Fig 9.
SPICE schematic diagram of LED booster with two 1 W LEDS in series as a load
Figure 10 depicts SPICE simulation for this use case. The output voltage is about 6.32 V
The peak current in the diode reaches about 0.7 A.
4
0.9
(1)
V
(V)
I
(A)
3
0.7
(2)
(3)
2
0.5
1
0.3
0
0.1
-1
0
0.2
0.4
0.6
0.8
t (μs)
1
-0.1
(1) VSW switching node voltage, green
(2)
ID1 diode current, red
(3) VO output voltage, blue
Fig 10. SPICE simulation of a load with two white LEDs in series
Figure 11 shows a scope trace measured at the voltage booster demo PCB. The
measurements conditions are: 2.5 V input voltage supply and an output load of a single
1 W white LED. Due to the small voltage boosting factor, the duty cycle is small. This
means the on-time for the switch in the controller IC is quite short compared to the cycle
time. The switching frequency proves to be about 1.4 MHz as specified for the LT1618.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Fig 11. SPICE simulation of the switching node captured from the LED booster demo
PCB for VI = 2.5 V and a single 1 W white LED load.
3.2 Thermal performance of the DFN and DSN Schottky diodes
Table 1 shows a comparison of Surface-Mounted Devise (SMD) packages for the thermal
performance. Very small DSN0603-2 (SOD962) chip scale package, with a length of
0.6 mm, a width and a height of 0.3 mm, can provide a similar Ptot as much bigger
standard SMD packages SOD523 or SOD323. 325 mW can be dissipated on a standard
footprint at an ambient temperature of 25°C. With 1 cm2 cathode pad, even 0.525 W can
be achieved.
The DFN1006D-2 (SOD882D) plastic package which measures 1 mm 0.6 mm 0.37
mm, can dissipate 370 mW on the standard or minimum footprint. With 1 cm2 cathode pad
Ptot of up to 735 mW is reached.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Products in DFN2020 packages can be found in many tablet backlight applications where
a bigger power is provided for the LED strings of the LCD display. The 2 mm 2 mm
packages have a very low thermal resistance from junction to solder point of
Rth(j-sp) = 12 K/W.
The thermal coupling of the crystal to the solder point is very good for leadless packages
because of the direct thermal path from the die into the PCB. Thermal resistance from
junction to solder point is listed in the first column of Table 1. If a low Rth(j-sp) is provided,
the thermal characteristic of the board has a dominant impact on the maximum power
dissipation that a component can dissipate. Modern electronic equipment such as
smartphones and mobile computing devices use multilayer board technology because the
system chips are ball-grid arrays or integrated circuits with a very small pitch of pins. More
than two layers are required to get all the connections realized.
Table 1.
Comparison of the package performance for Rth(j-sp), Ptot and IFSM
Package
Rth(j-sp)max
(K/W)
Ptot
Ptot
1 cm2 cathode pad, standard
(mW)
footprint,
(mW)
IFSM
8.3 ms
square wave
(A)
DSN0603-2 (SOD962)
40
525
325
6.5
DFN1006-2 (SOD882)
50
660
340
3.0
DFN1006D-2 (SOD882D) 25
735
370
6.0
DFN1608D-2 (SOD1608)
20
895
415
5.0
DFN2020-3 (SOT1061)
12
960
500
17.0
SOD523
75
-
310
6.0
SOD323
90
490
320
9.0
SOD323F
55
830
360
9.0
3.3 Thermal performance of the demo PCB
The current in the Schottky diode of the DC-to-DC step-up converter is depicted on
Figure 10 by curve 2 (red). The curve for the power dissipation looks very similar to the
one shown on Figure 12. A linear decrease of the current can be seen for the time the
diode conducts in forward direction and while the low-side switch in the controller IC is
switched on, a low reverse current leads to a small loss only. The average power
dissipation for the use case with a 1 W white LED load and a 2 V input voltage is about
125 mW. The average power of the pulse is Ppulse = 235 mW. The duty cycle is 0.53 from
the SPICE simulation. Zth(j-a) for this duty cycle on a standard footprint and a pulse width
of about 0.4 s can be extrapolated to roughly 130 K/W. The temperature increase can be
calculated with:
T = T j – T amb = Z th  j – a   P pulse = 31K
For an ambient temperature of 25°C, Tj of the Schottky diode should become 56 °C. This
calculation disregards additional heating from other components on the board. The major
contribution comes from the controller IC on the PCB LT1618. It has power losses of
about 105 mW.
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Leadless Schottky diodes in a DC-to-DC step-up converter
For a more accurate evaluation of the thermal behavior of the demo PCB, the board was
simulated with Mentor Graphics Corporation software FloTHERM. Models of the board
and the components were defined and the two mounting options with a Schottky diode in
packages DSN0603-2 (SOD962) and DFN1006-2 (SOD882) have been calculated. The
additional heat dissipation of the controller IC was modeled as well.
Fig 12. Power dissipation curve for a PMEG2005BELD with a 1 W white LED load and
2 V input voltage
Table 2 shows thermal simulation results, these are the junction temperatures for a power
dissipation of 125 mW and the maximum power that can be dissipated if the junction
temperature shall not exceed 125 °C and 150 °C. The junction temperature of the
Schottky diode is about 10 degree higher than the result of the calculation in which the
diode is the only component dissipating heat into the PCB. However on the PCB there are
significant power losses generated by the controller IC.
Table 2.
FloTHERM results for the DC-to-DC booster demo PCB, junction temperature for
Ptot = 125 mW and the maximum power dissipation for Tj = 125 °C and 150 °C for
Tamb = 25 °C
DFN1006-2
(SOD882)
AN11470
Application note
DSN0603-2
(SOD962)
P = 125 mW
Tj = 65 °C
Tj = 66 °C
Tj = 125 °C
Ptot = 405 mW
Ptot = 399 mW
Tj = 150 °C
Ptot = 520 mW
Ptot = 510 mW
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 22 April 2014
© NXP B.V. 2014. All rights reserved.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Figure 13 shows the temperature profile of the demo PCB for the area around the
Schottky diode. The big block left to the diode is the inductor. The left side shows the
thermal result for the PMEG2005AESF (DSN0603-2 package) and the PMEG2005BELD
(DFN1006D-2 package). The simulation shows the interesting fact that the hot spot for the
chip-scale diode is located at the anode side, whereas the part in the plastic package has
its hot spot at the cathode side. This difference should be considered for an optimized
thermal design with these package types. Thermal pads work most efficiently if they are
placed adjacent to the hotter pad for the chosen package to ensure the best thermal path
for the heat from the crystal into the board. For the demo board the big pad of the inductor
and the bulky metal block of this component create a good heat sink for the DSN0603-2
(SOT962) scenario. The inductor appears green in the 3D image below on Figure 13. This
means that it gets warmer compared to the other assembly option with the DFN1006-2
(SOD882) part. This explains why the temperatures for Ptot of 125 mW are very close for
the two scenarios, although the thermal performance of a DFN1006D-2 (SOD882D)
package is better compared to DSN0603-2 (SOD962).
Ptot = 125 mW
(1) left: DSN0603-2 (SOD962) package
(2) right: 1006D-2 (SOD882D) package
Fig 13. Temperature profile of the area around the Schottky diode on the LED booster
demo PCB.
Figure 14 shows a thermal diagram of the whole demo PCB derived from the FloTHERM
simulation. The LT1618 controller IC sticks out as a yellow rectangular area.
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Application note
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Rev. 1 — 22 April 2014
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Leadless Schottky diodes in a DC-to-DC step-up converter
(1) left: DSN0603-2 (SOD962) package
(2) right: 1006D-2 (SOD882D) package
Fig 14. Thermal diagram of the demo PCB with Ptot = 125 mW dissipated from the
Schottky diodes and a Ptot = 100 mW from the controller IC.
Figure 15 shows an infrared picture taken from an LED booster demo PCB run with an
input voltage of 2 V. As output load a 1 W white LED is connected. In this case the
temperature of the PMEG2005BELD reaches 57.4 °C. Because Tj is about 5 to 10 degree
warmer than the case, this fits quite well to the simulation results of the FloTHERM tool.
Fig 15. Infrared photograph of the LED booster PCB, area of the Schottky diode, VI = 2 V,
load is 1 W white LED, with PMEG 2005BELD
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Leadless Schottky diodes in a DC-to-DC step-up converter
3.4 NXP Schottky portfolio in DFN and DSN packages
Table 3 below shows extensive portfolio of NXP Semiconductors Schottky diodes in
DFN1006 packages. The maximum reverse voltage ranges from 20 V to 60 V. The
maximum forward current rating current has a range from 200 mA up to 1 A. For most of
the products two different package heights can be supported. The typical dimensions of
DFN1006-2 package are 1 mm 0.6 mm 0.48 mm. The DFN1006D-2 (SOD882D)
package has a typical height of 0.37 mm only, the length and width is identical to the
standard DFN1006-2 (SOD882) package. The Schottky diodes with a type name ending
with “ELD” have the package with the reduced height.
PMEG2005BELD and PMEG2010BELD have a benchmarking low forward voltage for this
1 mm 0.6 mm package type. VFmax for a forward current of 0.5 A is 0.44 V only.
Table 3.
NXP Semiconductors Schottky diodes portfolio in DFN1006-2 (SOD882) and
DFN1006D-2 (SOD882D)
Schottky diode type
VRmax
(V)
IFmax
(A)
IRmax @ VRmax
(mA)
VFmax @ VRmax
(V)
PMEG3002AEL
30
0.2
0.05
0.48
PMEG3002AELD
30
0.2
0.05
0.48
PMEG4002EL
40
0.2
0.01
0.6
PMEG4002ELD
40
0.2
0.01
0.6
PMEG2005BELD
20
0.5
0.2
0.39
PMEG2005AEL
20
0.5
1.5
0.44
PMEG2005AELD
20
0.5
1.5
0.44
PMEG2005EL
20
0.5
0.03
0.5
PMEG2005ELD
20
0.5
0.03
0.5
PMEF3005EL
30
0.5
0.5
0.5
PMEG3005ELD
30
0.5
0.5
0.5
PMEG2010BELD
20
1.0
0.2
0.49
PMEG6002ELD
60
0.2
0.1
0.6
Table 4 lists NXP Semiconductors Schottky diodes in a Wafer-Level Chip-Scale Package
(WLCSP) technology. The DSN0603-2 (SOD962) package has the dimensions of
0.6 mm 0.3 mm 0.3 mm only. This package is introduced for ESD diodes to a big
extend in the mobile communication market already. It allows a further miniaturization of
designs and a further step to achieve a higher power density in medium power
applications. Due to the fact that the crystal size is identical with the dimensions of the
package, a very good electrical performance can be achieved.
NXP Semiconductors offers two types of 20 V Schottky diodes in DSN0603-2 (SOD962).
The PMEG2002AESF is optimized for a low forward voltage drop. VF for IF = 200 mA or
IF = 500 ma is typically 420 mV only. The PMEG2002ESF is optimized for a low reverse
current. Maximum IR at Tj = 25 °C is 3.5 A only. PMEG2005AESF provides maximum VF
of 0.6 V for IF = 0.5 A. Schottky diodes for a maximum reverse voltage of 30 V and 40 V
are listed as well.
These new WLCSP Schottky diodes enable a further step of miniaturization in mobile
communication devices.
AN11470
Application note
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Rev. 1 — 22 April 2014
© NXP B.V. 2014. All rights reserved.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Table 4.
NXP Schottky diodes portfolio in DSN0603-2 (SOD962)
Schottky diode type
VR max
(V)
IF max
(A)
IR typ
@VR max
(A)
VF max
@VR max
(V)
PMEG2002AESF
20
0.2
45
0.42
PMEG2002ESF
20
0.2
4
0.49
PMEG2005AESF
20
0.5
45
0.55
PMEG2005ESF
20
0.5
4
0.6
PMEG3002AESF
30
0.2
100
0.48
PMEG3002ESF
30
0.2
10
0.55
PMEG3005AESF
30
0.5
100
0.63
PMEG4002ESF
40
0.2
10
0.6
PMEG4002AESF
40
0.2
95
0.53
The package type DFN1608D-2 (SOD1608) has become very popular in backlight
application for mobile devices such as smartphones. The dimensions of this package are
1.6 mm 0.8 mm 0.37 mm. NXP Semiconductors offers products with 20 V and 40 V
maximum reverse voltage as depicted on Table 5. The 20 V types are low VF versions,
whereas 40 V products are optimized for a low leakage current.
Table 5.
NXP Schottky diodes portfolio in DFN1608D-2 (SOD1608)
Schottky diode type
VR max
(V)
IF max
(A)
IR max
@VR max
(mA)
VF max
@VR max
(V)
PMEG2015EPK
20
1.5
0.9
0.42
PMEG4015EPK
40
1.5
0.03
0.61
PMEG2020EPK
20
2.0
0.9
0.45
PMEG4020EPK
40
2.0
0.03
0.66
Table 6 shows NXP Semiconductors Schottky diode products in the DFN2020-3
(SOT1061) package. It has the dimensions of 2 mm 2 mm 0.62 mm. This package
type can often be found in backlight applications for tablet PCs. The reverse voltage
ranges from 20 V up to 60 V. This allows to find a suitable diode for the applied LED string
length. The maximum forward current is 2 A for the single diode types and 1 A or 2 A for
the dual diode types.
AN11470
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Rev. 1 — 22 April 2014
© NXP B.V. 2014. All rights reserved.
20 of 25
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NXP Semiconductors
Leadless Schottky diodes in a DC-to-DC step-up converter
Table 6.
NXP Schottky diodes portfolio in DFN2020-3 (SOT1061)
Schottky diode type
VR max
(V)
IF max
(A)
IR max
@VR max
(mA)
VF max
@VR max
(V)
PMEG2020EPA
20
2.0
1.9
0.42
PMEG3020EPA
30
2.0
2.5
0.47
single diode
PMEG4020EPA
40
2.0
0.1
0.535
PMEG6020EPA
60
2.0
0.25
0.575
PMEG4010CPA
40
1.0
0.05
0.5
PMEG6010CPA
60
1.0
0.06
0.54
PMEG2020CPA
20
2.0
1.0
0.42
PMEG3020CPA
30
2.0
2.0
0.44
double diode, common cathode
4. Summary
NXP Semiconductor Schottky diodes in leadless packages such as DFN1006-2,
DFN1608-2 and DFN2020-3 and the chip-scale packages DSN0603-2 and DFN1006-2
allow more compact designs because of their good electrical and thermal performance.
This enables to make designs with an increased power density where more power can be
handled on a small mounting area. In modern electronic designs such as ultrabooks,
tablet PCs and smartphones, space constraints are getting more and more tough. Due to
the fact that all these applications are battery-driven, energy efficiency is an important
selection criterion as well. Low forward voltages of the Schottky diodes help reducing
losses.
The DC-to-DC step-up converter demo PCB described in this document proves that NXP
semiconductors small Schottky diodes can replace bigger packages very well without
compromising on performance, energy efficiency and reliability.
5. Appendix
In this appendix further information is given about the LED voltage booster demo PCB.
Figure 16 shows the place plan with the component values. Figure 17 depicts the top
layer of the board. The bottom layer is a solid copper ground plane.
AN11470
Application note
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Rev. 1 — 22 April 2014
© NXP B.V. 2014. All rights reserved.
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Leadless Schottky diodes in a DC-to-DC step-up converter
Fig 16. Place plan of the LED voltage booster PCB with component values
AN11470
Application note
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Rev. 1 — 22 April 2014
© NXP B.V. 2014. All rights reserved.
22 of 25
AN11470
NXP Semiconductors
Leadless Schottky diodes in a DC-to-DC step-up converter
Fig 17. Top layer of the LED voltage booster PCB
6. References
[1]
AN11470
Application note
Application Note AN11119 Medium power small-signal MOSFETs in DC-to-DC
conversion.
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 22 April 2014
© NXP B.V. 2014. All rights reserved.
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Leadless Schottky diodes in a DC-to-DC step-up converter
7. Legal information
7.1
Definitions
Draft — The document is a draft version only. The content is still under
internal review and subject to formal approval, which may result in
modifications or additions. NXP Semiconductors does not give any
representations or warranties as to the accuracy or completeness of
information included herein and shall have no liability for the consequences of
use of such information.
7.2
Disclaimers
Limited warranty and liability — Information in this document is believed to
be accurate and reliable. However, NXP Semiconductors does not give any
representations or warranties, expressed or implied, as to the accuracy or
completeness of such information and shall have no liability for the
consequences of use of such information. NXP Semiconductors takes no
responsibility for the content in this document if provided by an information
source outside of NXP Semiconductors.
In no event shall NXP Semiconductors be liable for any indirect, incidental,
punitive, special or consequential damages (including - without limitation - lost
profits, lost savings, business interruption, costs related to the removal or
replacement of any products or rework charges) whether or not such
damages are based on tort (including negligence), warranty, breach of
contract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, NXP Semiconductors’ aggregate and cumulative liability towards
customer for the products described herein shall be limited in accordance
with the Terms and conditions of commercial sale of NXP Semiconductors.
Right to make changes — NXP Semiconductors reserves the right to make
changes to information published in this document, including without
limitation specifications and product descriptions, at any time and without
notice. This document supersedes and replaces all information supplied prior
to the publication hereof.
Suitability for use — NXP Semiconductors products are not designed,
authorized or warranted to be suitable for use in life support, life-critical or
safety-critical systems or equipment, nor in applications where failure or
malfunction of an NXP Semiconductors product can reasonably be expected
to result in personal injury, death or severe property or environmental
damage. NXP Semiconductors and its suppliers accept no liability for
inclusion and/or use of NXP Semiconductors products in such equipment or
applications and therefore such inclusion and/or use is at the customer’s own
risk.
Applications — Applications that are described herein for any of these
products are for illustrative purposes only. NXP Semiconductors makes no
representation or warranty that such applications will be suitable for the
specified use without further testing or modification.
AN11470
Application note
Customers are responsible for the design and operation of their applications
and products using NXP Semiconductors products, and NXP Semiconductors
accepts no liability for any assistance with applications or customer product
design. It is customer’s sole responsibility to determine whether the NXP
Semiconductors product is suitable and fit for the customer’s applications and
products planned, as well as for the planned application and use of
customer’s third party customer(s). Customers should provide appropriate
design and operating safeguards to minimize the risks associated with their
applications and products.
NXP Semiconductors does not accept any liability related to any default,
damage, costs or problem which is based on any weakness or default in the
customer’s applications or products, or the application or use by customer’s
third party customer(s). Customer is responsible for doing all necessary
testing for the customer’s applications and products using NXP
Semiconductors products in order to avoid a default of the applications and
the products or of the application or use by customer’s third party
customer(s). NXP does not accept any liability in this respect.
Export control — This document as well as the item(s) described herein
may be subject to export control regulations. Export might require a prior
authorization from competent authorities.
Evaluation products — This product is provided on an “as is” and “with all
faults” basis for evaluation purposes only. NXP Semiconductors, its affiliates
and their suppliers expressly disclaim all warranties, whether express, implied
or statutory, including but not limited to the implied warranties of
non-infringement, merchantability and fitness for a particular purpose. The
entire risk as to the quality, or arising out of the use or performance, of this
product remains with customer.
In no event shall NXP Semiconductors, its affiliates or their suppliers be liable
to customer for any special, indirect, consequential, punitive or incidental
damages (including without limitation damages for loss of business, business
interruption, loss of use, loss of data or information, and the like) arising out
the use of or inability to use the product, whether or not based on tort
(including negligence), strict liability, breach of contract, breach of warranty or
any other theory, even if advised of the possibility of such damages.
Notwithstanding any damages that customer might incur for any reason
whatsoever (including without limitation, all damages referenced above and
all direct or general damages), the entire liability of NXP Semiconductors, its
affiliates and their suppliers and customer’s exclusive remedy for all of the
foregoing shall be limited to actual damages incurred by customer based on
reasonable reliance up to the greater of the amount actually paid by customer
for the product or five dollars (US$5.00). The foregoing limitations, exclusions
and disclaimers shall apply to the maximum extent permitted by applicable
law, even if any remedy fails of its essential purpose.
7.3
Trademarks
Notice: All referenced brands, product names, service names and trademarks
are the property of their respective owners.
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 22 April 2014
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Leadless Schottky diodes in a DC-to-DC step-up converter
8. Contents
1
2
2.1
2.2
3
3.1
3.2
3.3
3.4
4
5
6
7
7.1
7.2
7.3
8
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Methods of DC-to-DC voltage up conversion . 4
Voltage up conversion based on charge pumps 4
Inductor-based voltage up converter . . . . . . . . 6
Demo PCB DC-to-DC step-up converter using
very small Schottky diodes . . . . . . . . . . . . . . 10
Hardware of the DC-to-DC step-up converter on
demo PCB . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Thermal performance of the DFN and DSN
Schottky diodes . . . . . . . . . . . . . . . . . . . . . . . 14
Thermal performance of the demo PCB . . . . . 15
NXP Schottky portfolio in DFN and DSN
packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Legal information. . . . . . . . . . . . . . . . . . . . . . . 24
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in section ‘Legal information’.
© NXP B.V. 2014.
All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: salesaddresses@nxp.com
Date of release: 22 April 2014
Document identifier: AN11470