nr885e an en

NR885E APPLICATION NOTE
NR885E
Application Note Rev.4.0
SANKEN ELECTRIC CO., LTD.
http://www.sanken-ele.co.jp
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.1
Rev.4.0
NR885E APPLICATION NOTE
Rev.4.0
CONTENTS
General Descriptions ----------------------------------------------------------------------- 3
1. Electrical Characteristics ------------------------------------------------------------- 4
1.1 Absolute Maximum Ratings ----------------------------------------------------- 4
1.2 Recommended Operating Conditions --------------------------------------- 4
1.3 Electrical Characteristics -------------------------------------------------------- 5
2. Block Diagram & Pin Functions ----------------------------------------------------- 7
2.1 Functional Block Diagram ------------------------------------------------------- 7
2.2 Pin Asignments & Functions --------------------------------------------------- 7
3. Example Application Circuit---------------------------------------------------------- 8
4. Allowable package power dissipation -------------------------------------------- 9
5. Package Outline ------------------------------------------------------------------------- 10
6. Operational Descriptions ------------------------------------------------------------- 11
6.1 PWM (Pulse Width Modulation) Output Control ------------------------- 11
6.2 Power Supply Stability ----------------------------------------------------------- 11
6.3 Over Current Protection (OCP) ------------------------------------------------ 12
6.4 Thermal Shutdown (TSD) ------------------------------------------------------- 12
6.5 Soft-Start ------------------------------------------------------------------------------ 12
6.6 ON and OFF the Regulator (Enable) ----------------------------------------- 13
7. Design Notes ----------------------------------------------------------------------------- 14
7.1 External Components ------------------------------------------------------------ 14
7.2 Pattern Design ---------------------------------------------------------------------- 19
7.3 Applied Design ---------------------------------------------------------------------- 22
IMPORTANT NOTICE ---------------------------------------------------------------------- 24
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Page.2
NR885E APPLICATION NOTE
Rev.4.0
Package
General Descriptions
 Exposed SOIC 8
Thermally enhanced 8-Pin package
The NR885E is synchronous buck regulator ICs
integrates High-side and Low-side power MOSFETs.
With the current mode control, ultra low ESR
capacitors such as ceramic capacitors can be used.
The ICs have protection functions such as
Over-Current Protection (OCP), Under-Voltage
Lockout (UVLO) and Thermal Shutdown (TSD). An
adjustable Soft-Start by an external capacitor
prevents the excessive inrush current at turn-on. The
ICs integrate phase compensation circuit which
reduces the number of external components and
simplifies the design of customer application. The
ON/OFF pin (EN Pin) turns the regulator on or off
and helps to achieve low power consumption
requirements. The NR885E is available in an 8-pin
SOIC package with an exposed thermal pad on the
back side.
Electrical Characteristics
Features & Benefits








Current mode PWM control
Up to 95% efficiency
Stable with low ESR ceramic output capacitors
Built-in protection function
Over Current Protection (OCP)
Thermal Shutdown (TSD)
Under Voltage Lockout (UVLO)
 Built-in phase compensation
 Adjustable Soft-Start with an external capacitor
 Turn ON/OF the regulator function
3A Continuous output current
Operating input range VIN = 4.5V~18V
Output adjustable VO= 0.8V~14V
Fixed 350kHz
Applications
 LCD TV / Blu-Ray / Set top box
 Green Electronic products
 Other power supply
Series Lineup
Product No.
NR885E
(1)
(2)
fSW
350kHz
VIN
4.5V to 18V
VO
(1)
0.8V to 14V
The minimum input voltage shall be either of 4.5V or VO+3V, whichever is higher.
The I/O condition limited by the Minimum on-time (TON(MIN)) is in fig. 2.
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Page.3
IO
(2)
3A
NR885E APPLICATION NOTE
Rev.4.0
1. Electrical Characteristics
1.1 Absolute Maximum Ratings
Table 1 Absolute maximum rating of NR885E
Parameter
DC input voltage
Units
VIN
20
V
Conditions
Glass-epoxy board mounting
in a 30×30mm.
(copper area in a 25×25mm)
Max TJ =150°C
(3)
PD
1.76
W
Junction temperature
(4)
TJ
40 to 150
°C
TS
40 to 150
°C
θJP
26
°C /W
θJA
71
°C /W
Thermal resistance
(junction-ambient air)
(4)
Ratings
Power dissipation
Storage temperature
Thermal resistance
(junction- Pin No. 4)
(3)
Symbol
Glass-epoxy board mounting
in a 30×30mm.
(copper area in a 25×25mm)
Limited by thermal shutdown.
The temperature detection of thermal shutdown is about 160°C
1.2 Recommended Operating Conditions
Operating IC in recommended operating conditions is required for normal operating of circuit functions shown in
Table 3 Electrical characteristics of NR885E.
Table 2 Recommended operating conditions of NR885E
Parameter
DC input voltage
Symbol
(5)
Ratings
Units
MIN
MAX
VIN
Vo+3
18
V
Io
0
3.0
A
Vo
0.8
14
V
(6)
DC output current
(7)
Output voltage
Top
85
°C
40
The minimum value of input voltage is taken as the larger one of either 4.5V or VO +3V.
In the case of VIN=VO +1~VO +3V , it is set to IO = Max. 2A
(6)
Recommended circuit refers to Typical Application Circuit (fig.5).
(7)
To be used within the allowable package power dissipation characteristics (fig. 6)
Ambient operating temperature
(7)
(5)
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Page.4
Conditions
NR885E APPLICATION NOTE
Rev.4.0
1.3 Electrical Characteristics
Electrical characteristics indicate specific limits, which are guaranteed when IC is operated under the measurement
conditions shown in the circuit diagram (fig. 1)
Table 3 Electrical characteristics of NR885E
Parameter
(Ta=25°C)
Symbol
Reference voltage
Output voltage temperature
coefficient
Ratings
MIN
TYP
MAX
VREF
0.784
0.800
0.816
⊿VREF/⊿T
―
±0.05
―
350
420
Switching frequency
fSW
280
Units
Test conditions
V
VIN = 12V,IO = 1.0A
V = 12V, IO = 1.0A
mV/°C IN
40°C to +85°C
VIN =12V, VO=3.3V,
kHz
IO=1A
VIN = 6.3V~18V,
mV
VO = 3.3V, IO = 1A
VIN = 12V, VO = 3.3V,
mV
IO = 0.1A~3.0A
Line regulation
(8)
VLine
―
50
―
Load regulation
(8)
VLoad
―
50
―
Over current protection
threshold
IS
3.1
―
6.0
A
Supply Current
IIN
―
6
―
mA
IIN(off)
0
―
10
μA
IEN/SS
6
10
14
μA
VSS=0V, VIN=12V
VSSH
―
3.0
―
V
VIN=12V
50
100
μA
VEN= 10V
VIN=12V
Shutdown Supply Current
VIN = 12V, VO = 3.3V
VIN= 12V
VEN=10kΩ pull up to VIN
VIN=12V, IO=0A,
VEN=0V
Source current
SS Pin
EN Pin
at low level
voltage
High level
voltage
Sink current
IEN
Threshold voltage
Max on-duty
(8)
VC/EH
0.7
1.4
2.1
V
DMAX
―
90
―
%
TON(MIN)
―
150
―
nsec
165
―
°C
20
―
°C
(8)
Minimum on-time
(9)
Thermal shutdown threshold (8)
TSD
151
temperature
Thermal shutdown
(8)
restart hysteresis
TSD_hys
―
of temperature
(8)
Guaranteed by design,not tested.
(9)
The I/O characteristic limited by the TON(MIN) is in the fig.2.
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Page.5
NR885E APPLICATION NOTE
R3
2
7
EN
C1
C10
C3
1
L1
3
BS
IN
SW
R5
8
C2
Rev.4.0
SS
NR 885 E
IO
R4
IIN
I EN
NC
4
VEN
C5
I Fset
GND
FB
VO
5
6
V SS
R6
V FB
C1+ C2: 22μF / 25V
C4+C5: 44μF / 25V
C10: 0.1μF / 25V
R3: 22Ω
R4: 8.2 kΩ, R5: 4.3kΩ (Vo=3.3V)
R6: 3.9kΩ
L1: 10μH
fig.1 Measurement circuit diagram
20
18
16
14
VIN[V]
VIN
C4
12
10
8
6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Vo[V]
fig. 2 The I/O characteristic of NR885E
Be effective only if output current is less than 100mA.
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Page.6
2.4
RL
NR885E APPLICATION NOTE
Rev.4.0
2. Block Diagram & Pin Functions
2.1 Functional Block Diagram
fig.3 Block diagram of NR885E
2.2 Pin Asignments & Functions
fig.4 Pin Assignments
Table4 Pin functions of NR885E
Pin No.
Symbol
1
BS
2
IN
3
SW
4
GND
5
FB
6
NC
7
EN
8
SS
Description
High-side Boost input.
BS supplies the drive for High-side Nch-MOSFET switch.
Connect a capacitor and a resistor between SW to BS.
Power input. VIN supplies the power to the IC.as well as the regulator switches
Power switching output.
SW supplies power to the output.
Connect the LC filter from SW to the output.
Note that a capacitor is required from SW to BS to supply the power the High-side switch
Ground
Connect the exposed pad to Pin No.4
Feedback input Pin to compare Reference Voltage.The feedback threshold is 0.8V.
To set the output voltage, FB Pin is required to connect between resistive voltage divider
R4 and R6.
No Connection.
Enable input.
Drive EN Pin high to turn on the regulator, low to turn it off.
Soft-Start control input.
To set the soft-start period, connect to a capacitor between GND.
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Page.7
NR885E APPLICATION NOTE
Rev.4.0
3. Example Application Circuit
Each ground of all components is connected as close as possible to the Pin No.4 at one point.
To help heat dissipation, connect a large copper plane to exposed pad on the back side of the package. The copper
plane is required for GND
C1, C2: 10μF / 25V
C4, C5: 22μF / 16V
C9: 0.1μF
C3: 0.1μF
R1: 100kΩ
R3: 22Ω
R4: 8.2 kΩ, R5: 4.3kΩ (Vo=3.3V)
R6: 3.9kΩ
fig. 5 Typical Application Circuit of NR885E
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Page.8
L1: 10μH
NR885E APPLICATION NOTE
Rev.4.0
4. Allowable package power dissipation
fig. 6 Allowable package powe disspation of NR885E
NOTES:
1) Glass-epoxy board mounting in a 30×30mm
2) copper area : 25×25mm
3) The power dissipation is calculated at the junction temperature 125 °C
4) Losses can be calculated by the following equation.
As the efficiency is subject to the input voltage and output current, it shall be obtained from the efficiency curve
and substituted in percent
5) Thermal design for D1 shall be made separately.
 100

PD  VO  I O 
 1
 x

VO: Output voltage
VIN : Input voltage
IO : Output current
ηx : Efficiency (%)
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Page.9
NR885E APPLICATION NOTE
Rev.4.0
5. Package Outline
 Exposed SOIC8 package
6.918(0.244)
5.893(0.232)
2.16±0.127
(0.085±0.005)
3.89 (0.153)
3.73 (0.147)
0.508(0.020)
0.330(0.130)
3.18±0.127
(0.120±0.005)
Bottom view
Top view
4.902±0.102
(0.193± 0.004)
1.448±0.05
(0.057±0.002)
1.27(0.050) BSC
0.102(0.004)
0.1524(0.006 )
0.000(0.000)
Front view
Side view
Notes:
1) Dimension is in millimeters, dimension in bracket is in inches.
2) Drawing is not to scale.
fig.7 Package outline
NR885E
SKYMW
Part Number
Lot Number
Y= last digit of the year (0-9)
M= Month (1-9, O, N, or D)
W= Week Code (1-3)
XXXX
Sanken Control Number
fig.8 Marking of NR885E
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Page.10
NR885E APPLICATION NOTE
Rev.4.0
6. Operational Descriptions
6.1 PWM (Pulse Width Modulation) Output Control
The NR885E consist of three blocks; two feedback loops (voltage control and current control) and one slope
compensation. The PWM is controlled with the current mode control by calculating the voltage feedback control, and
the current feedback control and the slope compensation signals. (fig.9) For the voltage feedback control, the output
voltage feed back to the PWM control. The error amplifier compares the output voltage divided by resistors with the
reference voltage VREF = 0.8V. For the current feedback control, the inductor current feed back to the PWM control.
The inductor current divided by Sense-MOSFET is detected with the current sense amplifier. To prevent sub-harmonic
oscillations, which is characteristic in current mode control, the slope of current control is compensated.
fig.9 Basic Structure of Chopper Type Regulator with PWM Control by Current Control
The NR885E start the switching operation when UVLO is released, or EN or SS Pin voltage exceeds the threshold.
Initially, it operates switching with minimum ON duty or maximum ON duty. The high-side switch (M1) is the
switching MOSFET that supplies output power. At first, the low-side switch (M2) turns ON and charges the boost
capacitor C3 that drives M1. When M1 is ON, as the inductor current is increased by applying voltage to SW Pin and
the inductor, the output of inductor current sense amplifier is also increased. Sum of the current sense amplifier output
and slope compensation signal is compared with the error amplifier output. When the summed signal exceeds the error
amplifier (Error Amp.) output voltage, the current comparator output becomes “High” and the RS flip-flop is reset.
When M1 turns OFF and M2 turns ON, the regenerative current flows through M2. In the case that an external SBD
(D1) is connected, the current also flows through D1.
In NR885E, the set signal is generated in each cycle and RS flip-flop is set. In the case that the summed signal does not
exceed the error amplifier (Error Amp.) output voltage, RS flip-flop is reset without fail by the signal from OFF duty
circuit.
6.2 Power Supply Stability
The phase characteristics of chopper type regulator are the synthesis of the internal phase characteristics of regulator IC,
the combination of output capacitor C4 (C5) and load resistance Rout. The internal phase characteristics of regulator
IC are generally determined by the delay time of control block and the phase characteristics of output error amplifier.
The phase delay due to the delay time of control block is very small and not problem in actual use. As the built-in
phase compensation for output error amplifier, in order to ensure stable operation, refer to “7.1.3 Output Capacitor
C4 (C5)” and “7.1.4 Output Voltage Set-up (FB Pin)” for setting the output voltage and output capacitor,
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Page.11
NR885E APPLICATION NOTE
Rev.4.0
The NR885E integrate the drooping type over-current
protection circuit. The peak current of switching transistor is
detected. When the peak current exceeds rated value, the
over-current protection limits the current by forcibly
shortening the ON time of transistor and decreasing the
output voltage. It prevents the current increment at low
output voltage by decreasing the switching frequency, if the
output voltage drops lower. When the over-current state is
released, the output voltage automatically returns.
Output Voltage Vo [V]
6.3 Over Current Protection (OCP)
Output Voltage Vo [V]
fig.10 OCP Characteristics of NR885E
6.4 Thermal Shutdown (TSD)
The thermal shutdown circuit detects the IC junction
temperature. When the junction temperature exceeds the
rated value (around 160°C), it shuts-down the output
transistor and turns the output OFF. If the junction
temperature falls below the thermal shutdown rated value by
around 20°C, the operation returns automatically.
* (Thermal Shutdown Characteristics) Notes
The circuit protects the IC against temporary heat generation.
It does not guarantee the operation including reliabilities
under the continuous heat generation conditions, such as
short circuit for a long time.
Output voltage
Rated
Protection
Temperature
Rated Restart
Temperature
Junction
Temperature
fig.11 TSD Characteristics of NR885E
By connecting a capacitor between Pin No.8 (SS) and Pin
No.4 (GND), Soft-Start operates when the power is supplied
to the IC. Output Voltage (Vo) is ramped up by the charge
voltage level of Css.
Time of Soft-Start can be calculated from the time constant
of charging Css.
A capacitor Css controls OFF period of PWM and then the
rise time is determined. The rise time t_ss and the delay time
t_delay are calculated in following equations.
In the case of operating IC without using Soft-Start function,
Pin No.8 is required for open.
[msec]
Time of Soft-Start
ソフトスタート時間 [ms]
出力起動時間
6.5 Soft-Start
100
10
1
0.1
0.01
0.0001
0.001
0.01
SSコンデンサCss [uF]
Pin SS capacitance
Css [uF]
fig.12 SS Pin capacitance CSS VS Soft-Start
tSS = CSS × (VSS2 – VSS1) / (ISS × VSS1)
VSS1 (0.9V) < SS Pin voltage < VSS2 (1.79V)
t_delay = CSS × VSS1 / ISS
SS Pin voltage < VSS1(th) = VSS1 (0.9V)
fig.13 Soft-Start operating description
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t_delay
t_ss
t_all
Page.12
0.1
NR885E APPLICATION NOTE
Rev.4.0
In the case of without or too small Css, IC starts up at the
time constant that output current limited by Is charges output
capacitor C4 (C5).
Following equation shows the time constant of start up by the
output capacitor C4 (C5) at no load.
t = (CO × VO) / IS
*At the start up with a load, load current is detracted from Is.
fig.14 Discharge time of SS capacitor
Pin SS Open Voltage: 3V
SS Discharge Capability: 500µA
The left graph shows the SS Pin voltage changing
time from 3V to 0V.
6.6 ON and OFF the Regulator (Enable)
EN Pin (Pin No.7) turns the regulator ON or OFF. When
drive EN under 1.4V (VENL), output is turned OFF (fig.15).
1.4V (VENL) can be achieved by connecting a bipolar
transistor in an open collector configuration.
When the external ON/OFF function isn't used, connect only
Pull-up resistor of 100kΩ between IN and EN, and use it as
the fig16.It starts when a VIN voltage is inputted.
.
2. IN
100kΩ
7.EN
NR885E
fig. 15 ON / OFF Control 1
2. IN
100kΩ
7.EN
NR885E
fig. 16 ON / OFF Control 2
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Page.13
NR885E APPLICATION NOTE
Rev.4.0
7. Design Notes
7.1 External Components
All components are required for matching to the condition of use.
7.1.1 Choke Coil L1
The choke coil L1 is one of the most important components in the chopper type switching regulators. In order to
maintain the stabilized regulator operation, the coil should be carefully selected so it must not enter saturation or over
heat excessively at any conditions. The selection points of choke coil are as follows:
a) The coil type is only required for switching regulator.
It is recommended not to use a coil for noise filer since it causes high heat generation due to high power dissipation.
From fig.17 show the selection range of inductance L to
prevent the sub-harmonic oscillations. As for the upper
limit of inductance L, the value is for reference, because it
may vary depending on input/output conditions and load
current.
The ripple portion of choke coil current ΔIL and the peak
current ILp are calculated from the following equations:
IL 
(VIN  Vo )  Vo
L  VIN  f
------ (1)
Inductance L [µH]
b) The sub-harmonic oscillations should be prevented.
Under the peak detection current control, the inductor current may fluctuate at a frequency that is an integer multiple
of switching operation frequency. This phenomenon is the known as sub-harmonic oscillation and this phenomenon
theoretically occurs in the peak detection current control mode. In order to stabilize the operation, the inductor
current compensation is made internally. The inductance corresponding to the output voltage should be selected.
Inductance L
Selection Range
Output Voltage Vo [V]
fig.17 Selection Range of Inductance L in NR885E
ILp 
IL
 Io
2
------ (2)
The ΔIL and ILp increase when the inductance of the choke
coil L becomes smaller.
If the inductance is too small, the regulator operation may be
unstable because the choke coil current fluctuates largely. It
is necessary to give attention to decreasing of the inductance
due to the magnetic saturation such as in overload and load
shortage.
High Inductance
Low Inductance
fig.18 Relationship between inductance and ripple current
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Page.14
NR885E APPLICATION NOTE
Rev.4.0
c) The coil should be of proper rated current.
The rated current of the choke coil should be higher than the maximum load current used. If the load current exceeds
the rated current of coil, the inductance decreases drastically and eventually enters into the saturation state. In this
status, it is necessary to give attention because the high-frequency impedance decreases and the excess current runs.
d) The magnetic noise should be minimized.
The open magnetic circuit type core like a drum type may generate noise in peripheral circuit due to the magnetic
flux passing outside of coil. Coils of closed magnetic circuit type core, such as toroidal type, EI type and EE type are
preferable.
7.1.2 Input Capacitor C1 (C2)
The input capacitor operates as a bypass capacitor of input circuit. It supplies the short current pulses to the regulator
during switching and compensates the input voltage drop. It should be connected close to the regulator IC. Even if the
rectifying capacitor of AC rectifier circuit is in input circuit, the input capacitor cannot be used as a rectifying capacitor
unless it is connected near IC.
The selection points of C1 (C2) are as follows:
a) The capacitor should be of proper breakdown voltage rating
b) The capacitor should have sufficient allowable ripple current rating
IIN
If the input capacitor C1 (C2) is used under the conditions of
excessive breakdown voltage or allowable ripple current, or
without derating, the regulator may become unstable and the
capacitor’s lifetime may be greatly reduced. The selection of
the capacitor C1 (C2) is required for the sufficient margins to
the ripple current. The effective value of ripple current Irms
that flows across the input capacitor is calculated from the
equation (3):
Irms  1.2 
VIN
IN
Ripple
current
C1 (C2)
Vo
 Io ------ (3)
Vin
fig. 19 C1 (C2) Current path
In the case of VIN = 20V, Io = 3A, Vo = 5V,
5
IIN3  0.9A
20
VIN
Irms  1.2 
1.V
The capacitor is required for the allowable ripple currentIN of
Ripple
0.9A or higher.
current
Ip
0
Iv
C1
Ton
T
fig.20 C1 (C2) Current Waveform
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Page.15
NR885E APPLICATION NOTE
Rev.4.0
7.1.3 Output Capacitor C4 (C5)
In the current control mode, the feedback loop which detects the inductor current is added to the voltage control mode.
The stable operation is achieved by adding inductor current to the feedback loop without considering the effect of
secondary delay factor of LC filter. It is possible to reduce the capacitance of LC filter that is needed to make
compensations for the secondary delay, and the stable operation is achieved even by using the low ESR capacitor
(ceramic capacitor).
The output capacitor C4 (C5) comprises the LC low-pass filter with choke coil L1 and works as the rectifying capacitor
of switching output. The current equal to ripple portion ΔIL of choke coil current charges and discharges the output
capacitor. In the same way as the input capacitor, the breakdown voltage and the allowable ripple current should be met
with sufficient margins.
The ripple current effective value of output capacitor is
calculated from the equation (4):
IL
IL
2 3
L1
Irms 
Vout
------ (4)
ESR
When ΔIL = 0.5A,
Irms 
0.5
≒ 0.14A
2 3
IL
fig. 21 C4 (C5) Current path
The output ripple voltage of regulator Vrip is determined by Vout
the product of choke current ripple L1
portion ΔIL (= C4 (C5)
discharge and charge current) and output capacitor C4 (C5)
Ripple
Io
equivalent series resistance ESR.
current
RL
It is necessary to select a capacitor with low equivalent series
resistance ESR in order to lower the output ripple voltage. As
C2
for general electrolytic capacitors of same product series, the
ESR shall be lower for products of higher capacitance with
same breakdown voltage, or of higher breakdown voltage
with same capacitance.
When ΔIL = 0.5A, Vrip = 40mV,
C4 ESR  40  0.5  80m
A capacitor with ESR of 80mΩ or lower should be selected.
Since the ESR varies with temperature and increases at low
temperature, it is required to check the ESR at the actual
operating temperatures. It is recommended to contact
capacitor manufacturers for the ESR value since it is peculiar
to every capacitor series.
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Page.16
RL
C4 (C5)
Therefore a capacitor with the allowable ripple current of
0.14A or higher is needed.
Vrip  IL  C4 ESR -----ESR(5)
Io
Ripple
current
C2 Current
Waveform
0
∆IL
fig. 22 C4 (C5) Current Waveform
NR885E APPLICATION NOTE
Rev.4.0
7.1.4 Output Voltage Set-up ( FB Pin )
The FB Pin is the feedback detection Pin that controls the
output voltage. It is recommended to connect close to the
output capacitor C4 (C5). If they are not close, the abnormal
oscillations may be caused by the poor regulation and the
increased switching ripple.
The setting of output voltage is achieved by connecting
between resistive voltage divider R4 (R5) and R6. Setting the
IFB to about 0.2mA is recommended.
(The target of IFB lower limit is 0.2mA, and the upper limit is
not defined. However, it is necessary to consider that the
circuit current shall increase according to the IFB value.)
R4 (R5), R6 and the output voltage VO are calculated from
the following equations:
IFB = VFB / R6
fig. 23 Detection and setting of output voltage
*VFB = 0.8V ± 2%
R4 + R5 = ( VO  VFB ) / IFB
R6 = VFB / IFB
VO = ( R4 + R5 ) × ( VFB / R6 ) + VFB
R6 is required to connect for the stable operation when set to VO = 0.8V.
Regarding the relation of input / output voltages, it is recommended that setting of the ON width of the SW Pin is more
than 200nsec
The PCB circuit traces of FB Pin, R4 (R5) and R6 are required for not parallel to the flywheel diode. The switching
noise may affect the detection voltage and the abnormal oscillation may be caused. Especially, it is recommended to
design the circuit trace short from FB Pin to R6.
7.1.5 External Bootstrap Diode for Low Input
Although the NR880E will work with input voltages lower than 6V, it is recommended to connect a diode between IN
Pin and BS Pin in order to enhance the efficiency (fig.24). Alternatively an external voltage source can be connected
through a diode to the BS Pin (fig.25).
NOTES:
1) The external voltage is required to be set from 5V to 6V.
2) In the case that the input voltage VIN is higher than 6V, the Bootstrap Diode must not be connected.
5V
2. IN
1.BS
1.BS
NR885E
NR885E
3.SW
3.SW
fig.24 Bootstrap Diode Connection 1
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fig.25 Bootstrap Diode Connection 2
Page.17
NR885E APPLICATION NOTE
Rev.4.0
7.1.6 Flywheel Diode D1
A flywheel diode can be connected to enhance the efficiency.
The flywheel diode D1 is for releasing the energy stored in the choke coil at switching OFF. It is required to use a
Schottky barrier diode for flywheel diode. If a general rectifying diode or a fast recovery diode is used, the IC may fail
to operate properly becase of applying reverse voltage due to the recovery and ON voltage. Since the output voltage
from the SW Pin (Pin No. 3) is almost equal to the input voltage, it is required to use the flywheel diode with the
reverse breakdown voltage of equal or higher than the input voltage.
It is recommended not to use ferrite beads for flywheel diode.
7.1.7 Output Voltage VO and Output Capacitor C4 (C5)
From Table 6 show the comparison of output voltage and output capacitor, for maintaining the IC stable operations, for
reference. ESR of Electrolytic Capacitor is required from 100m Ω to 200mΩ.
Regarding the inductance L, it is recommended to select it according to 7.1.1 Choke Coil L1.
Table 6 NR885E (fSW =350kHz) VO and C4 (C5) Comparison
C4 (C5) (µF)
VO(V)
Electrolytic Capacitor
(ESR≒100mΩ)
Ceramic Capacitor
1.2
33 to 100
47 to 330
1.8
22 to 100
47 to 470
3.3
10 to 68
20 to 180
5
4.7 to 47
4.7 to 100
9
3.3 to 22
2.2 to 47
12
3.3 to 22
2.2 to 33
14
2.2 to 22
2.2 to 33
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Page.18
NR885E APPLICATION NOTE
Rev.4.0
7.2 Pattern Design
7.2.1 High Current Line
High current paths in the circuit are marked as bold lines in the circuit diagram below. These paths are required for
wide and short trace as possible.
fig.26 Circuit Diagram
7.2.2 Input / Output Capacitors
The input capacitor C1 (C2) and the output capacitor C4 (C5) are required to connect to the IC as short as possible. If
the rectifying capacitor for AC rectifier circuit is in the input side, it can be also used as an input capacitor. However,
if it is not close to the IC, the input capacitor is required to be connected in addition to the rectifying capacitor. A
similar care should be taken when designing pattern for other capacitors.
fig. 27 Proper Pattern Example
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fig. 28 Improper Pattern Example
Page.19
NR885E APPLICATION NOTE
Rev.4.0
7.2.3 PCB Layout Example & Recommended Land Pattern
fig.29 Front Side: Component Side (double sided board)
fig.30 Back Side: GND Side (double sided board)
Circuit Diagram of Demo-board PCB
NR885E
fig.31 Demo-board circuit-diagram
As for the part number of the demonstration board "circuit-diagram", a circuit board concerned doesn't partly fit
each other with the above application circuit example and so on for NR110, NR120 and the NR880 series
common use. Approve it in advance. C9, R9 and C10 aren't used in the NR880 series. And, D1, D2, R3, R8,
R10,C11 and C12 are options.
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Page.20
NR885E APPLICATION NOTE
Rev.4.0
NOTES:
Size of the PCB is about 60mm×60mm
0.61 (0.024)
1.27 (0.050)
1.60 (0.063)
2.35 (0.092)
5.40 (0.213)
3.24 (0.127)
NOTES:
1) Dimension is in millimeters, dimension in bracket is in inches.
2) Drawing is not to scale.
fig.32 Recommended land pattern
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Page.21
NR885E APPLICATION NOTE
Rev.4.0
7.3 Applied Design
7.3.1 Spike Noise Reduction(1)
RBS
・The addition of the BS serial resistor
The “turn-on switching speed” of the internal
Power-MOSFET can be slowed down by inserting RBS
(option) of the fig33.It is tendency that Spike noise becomes
small by reducing theswitching-speed. Set up 22-ohm as an
upper limit when you use RBS.
*Attention
1) When the resistance value of RBS is enlarged by mistake
too much, the internal power-MOSFET becomes an
under-drive,it may be damaged worst.
2) The “defective starting-up” is caused when the resistance
value of RBS is too big.
*The BS serial resistor RBS is R3 in the Demonstration
Board.
1.BS
NR885E
fig.33 The addition of the BS serial resistor
7.3.2 Spike Noise Reduction(2)
2.IN
・The addition of the Snubber circuit
In order to reduce the spike noise, it is possible to compensate
the output waveform and the recovery time of diode by
connecting a capacitor and resistor parallel to the freewheel
diode (snubber method). This method however may slightly
reduce the efficiency.
* For observing the spike noise with an oscilloscope, the probe
lead (GND) should be as short as possible and connected to the
root of output capacitor. If the probe GND lead is too long, the
lead may act like an antenna and the observed spike noise may be
much higher and may not show the real values.
*The snubber circuit parts are C12 and R10.
3.SW
NR885E
4.GND
≒10Ω
*Option
≒1000pF
fig.34 The addition of the Snubber circuit
7.3.3 Attention about the insertion of the bead-core
fig.35
In the area surrounded by the red dotted line within the fig35, don't insert the bead-core such as Ferrite-bead.
As for the pattern-design of printed-circuit-board, it is recommended that the parasitic-inductance of wiring-pattern is
made small for the safety and the stability.
When bead-core was inserted, the inductance of the bead-core is added to parasitic-inductance of the wiring-pattern.
By this influence, the surge-voltage occurs often, or , GND of IC becomes unstable, and also, negative voltage occurs
often.
Because of this, faulty operation occurs in the IC. The IC has the possibility of damage in the worst case.
About the Noise-reduction, fundamentally, Cope by "The addition of CR snubber circuit" and "The addition of BS serial
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Page.22
NR885E APPLICATION NOTE
Rev.4.0
resistor".
7.3.4 Reverse Bias Protection
A diode for reverse bias protection may be required between
input and output in case the output voltage is expected to be
higher than the input Pin voltage (a common case in battery
charger applications).
2. IN
3.SW
NR885E
fig.36 Reverse bias protection diode
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Page.23
NR885E APPLICATION NOTE
Rev.4.0
IMPORTANT NOTICE
 The contents in this document are subject to changes, for improvement and other purposes, without notice.
Make sure that this is the latest revision of the document before use.
 Application and operation examples described in this document are quoted for the sole purpose of reference
for the use of the products herein and Sanken can assume no responsibility for any infringement of
industrial property rights, intellectual property rights or any other rights of Sanken or any third party which
may result from its use.
 Although Sanken undertakes to enhance the quality and reliability of its products, the occurrence of failure
and defect of semiconductor products at a certain rate is inevitable. Users of Sanken products are requested
to take, at their own risk, preventative measures including safety design of the equipment or systems against
any possible injury, death, fires or damages to the society due to device failure or malfunction.
 Sanken products listed in this document are designed and intended for the use as components in general
purpose electronic equipment or apparatus (home appliances, office equipment, telecommunication
equipment, measuring equipment, etc.).
When considering the use of Sanken products in the applications where higher reliability is required
(transportation equipment and its control systems, traffic signal control systems or equipment, fire/crime
alarm systems, various safety devices, etc.), please contact your nearest Sanken sales representative to
discuss, prior to the use of the products herein.
The use of Sanken products without the written consent of Sanken in the applications where extremely high
reliability is required (aerospace equipment, nuclear power control systems, life support systems, etc.) is
strictly prohibited.
 In the case that you use our semiconductor products or design your products by using Sanken
semiconductor products, the reliability largely depends on the degree of derating to be made to the rated
values. Derating may be interpreted as a case that an operation range is set by derating the load from each
rated value or surge voltage or noise is considered for derating in order to assure or improve the reliability.
In general, derating factors include electric stresses such as electric voltage, electric current, electric power
etc., environmental stresses such as ambient temperature, humidity etc. and thermal stress caused due to
self-heating of semiconductor products. For these stresses, instantaneous values, maximum values and
minimum values must be taken into consideration.
In addition, it should be noted that since power devices or IC’s including power devices have large
self-heating value, the degree of derating of junction temperature affects the reliability significantly.
 When using the products specified herein by either (i) combining other products or materials therewith or
(ii) physically, chemically or otherwise processing or treating the products, please duly consider all possible
risks that may result from all such uses in advance and proceed therewith at your own responsibility.
 Anti radioactive ray design is not considered for the products listed herein.
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out of Sanken’s distribution network.
 The contents in this document must not be transcribed or copied without Sanken’s written consent.
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Page.24