nr887d an en

NR887D APPLICATION NOTE
NR887D
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
NR887D 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 ---------------------------------------------------------------------- 21
IMPORTANT NOTICE ---------------------------------------------------------------------- 23
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.2
NR887D APPLICATION NOTE
Rev.4.0
Package
General Descriptions
 DIP 8
The NR887D 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 NR887D is available in an 8-pin
DIP package.
BS
SS
IN
EN
SW
NC
GND
FB
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
2A Continuous output current
Operating input range VIN = 4.5V~18V
Output adjustable VO= 0.8V~14V
Fixed 500kHz frequency
Applications
 LCD TV / Blu-Ray / Set top box
 Green Electronic products
 Other power supply
Series Lineup
Product No.
fO
VIN
VO
(1)
NR887D
500kHz
4.5V to 18V
0.8V to 14V
The minimum input voltage shall be either of 4.5V or VO+3V, whichever is higher.
(2)
The I/O condition is limited by the Minimum on-time (TON(MIN)).
(1)
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.3
IOUT
(2)
2A
NR887D APPLICATION NOTE
Rev.4.0
1. Electrical Characteristics
1.1 Absolute Maximum Ratings
Table 1 Absolute maximum rating of NR887D
Parameter
DC input voltage
(3)
(4)
Symbol
Ratings
Units
VIN
20
V
Remarks
Glass-epoxy board mounting
in a 70mm×60mm.
(copper area in a 1310mm2)
Max Tj =150°C
Power dissipation
(3)
Pd
1.85
W
Junction temperature
(4)
Tj
40 to 150
°C
Storage temperature
Thermal resistance
(junction- Pin No. 4)
Tstg
40 to 150
°C
θj-c
25
°C /W
Thermal resistance
(junction-ambient air)
θj-a
67
°C /W
Glass-epoxy board mounting
in a 70mm×60mm.
(copper area in a 1310mm2)
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 NR887D.
Table 2 Recommended operating conditions of NR887D
Parameter
DC input voltage
Symbol
Ratings
Units
MIN
MAX
VIN
VO+3
18
V
(7)
IOUT
0
2.0
A
VO
TOP
0.8
14
V
(7)
40
85
°C
(5)
Remarks
(6)
DC output current
Output voltage
Ambient operating temperature
(5)
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 IOUT=1A MAX and it is possible to apply the controlled VIN by the
Max-on-duty. Refer the following equation. VIN = VO / 0.9(typ)
(6)
Recommended circuit refers to Typical Application Circuit (fig.4).
To be used within the allowable package power dissipation characteristics (fig. 5)
(7)
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.4
NR887D 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 NR887D series
Parameter
Symbol
Reference voltage
Output voltage temperature
coefficient
Ratings
MIN
TYP
MAX
VREF
0.784
0.800
0.816
⊿VREF/⊿T
―
±0.05
―
η
―
90
―
500
600
(8)
Efficiency
(Ta=25°C)
Operating frequency
fO
400
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,
%
IO=1A
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~2.0A
Line regulation
(9)
VLine
―
50
―
Load regulation
(9)
VLoad
―
50
―
Over current protection
starting current
IS
3.1
―
6.0
A
Quiescent current 1
IIN
―
6
―
mA
Quiescent current 2
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
SS Pin
EN Pin
Flow-out current at
low level voltage
High level
voltage
Sink current
IEN
VC/EH
0.7
1.4
2.1
V
Max on-duty
(9)
DMAX
―
90
―
%
Minimum on-time
Thermal shutdown
temperature
Thermal shutdown
restart hysteresis
of temperature
(9)
TON(MIN)
―
150
―
nsec
(9)
TSD
151
165
―
°C
(9)
TSD_hys
―
20
―
°C
(8)
Threshold voltage
threshold
Efficiency is calculated by the following equation.
(9)
Guaranteed by design,not tested.
Copy Right: SANKEN ELECTRIC CO., LTD.
η(%)=
Page.5
VO・IO
VIN・II N
×100
VIN = 12V, VO = 3.3V
VIN= 12V
VEN=10kΩ pull up to VIN
VIN=12V, IO=0A,
VEN=0V
NR887D APPLICATION NOTE
EN
8
C1
IIN
IEN
VIN
SS
SW
R1
NR887D
IO
C2
FB
I Fset
4
VEN
L1
3
BS
GND
VIN
C3
1
2
7
Rev.4.0
NC
5
VFB
6
VSS
C1:22μ F/25V
C2:44μ F/25V
C3: 0.1μ F /25V
L1:10μ H
R1: 5 kΩ
R2: 1.6 kΩ
fig.1 Measurement circuit diagram
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.6
R2
VO
RL
NR887D APPLICATION NOTE
Rev.4.0
2. Block Diagram & Pin Functions
2.1 Functional Block Diagram
VIN
2
Σ
OSC
R1
7
3
EN
ON/
OFF
VREF
5
IN
Current
Sense
6
Drive
REG
OCP
P.REG
C1
C2
1
BS
R3
C10
0.8V
PWM
LOGIC
4
L1
3
SW
D1
TSD
R5
VO
C4
C5
R4
Compensation
UVLO
6
8
7
0.8V
NC
5
FB
Error Amp.
R6
SS
1
GND
1
4
SS
8
C9
fig.2 Block diagram of NR887D
2.2 Pin Asignments & Functions
BS
SS
IN
EN
SW
NC
GND
FB
fig.3 Pin Assignments
Table4 Pin functions of NR887D
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.
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.7
NR887D APPLICATION NOTE
Rev.4.0
3. Example Application Circuit
Each ground of all components is connected as close as possible to the Pin No.1 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
R3
VIN
R1
VIN_s
7
C1
C2
1
BS
C10
SW
SS
GND
4
Vo_s
C7
FB
R5
C4 C5
5
R7
NC
6
R4
C9
Option
R6
D1
SW
C1, C2: 10μF / 25V
C4, C5: 22μF / 16V
C7: 1000pF
C9: 0.1μF
C10: 0.1μF
R1: 100kΩ
R3: 22Ω
R4: 1.1kΩ, R5: 3.9kΩ (Vo=3.3V)
R6: 1.6kΩ
R7: 10Ω
fig. 4 Typical Application Circuit of NR887D
Copy Right: SANKEN ELECTRIC CO., LTD.
Vo
L1
3
NR887D
8
GND
2
IN
EN
Page.8
L1: 10μH
GND
NR887D APPLICATION NOTE
Rev.4.0
4. Allowable package power dissipation
1.8
1.6
許容損失 PD[W]
Power Dissipation
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-25
0
25
50
周囲温度 Ta[℃]
Ambient Temperature
75
100
125
fig. 5 Allowable package powe disspation of NR887D
NOTES:
1) Glass-epoxy board mounting in a 70mm × 60mm
2) copper area : 1310mm2
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 (%)
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.9
NR887D APPLICATION NOTE
5. Package Outline
 DIP8
Notes:
1) Dimension is in millimeters.
2) Drawing is not to scale.
fig.6 Package outline
*1
NR887D
SK *2
*3
*1. Product number
*2. Lot number (three digit)
1st letter : The last digit of the year
2nd letter : Month
January to September : 1 to 9
October : O
November : N
December : D
3rd letter : manufacturing week
First week to 5th week : 01 to 05
*3. Control number (four digit)
fig.7 Marking of NR887D
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.10
Rev.4.0
NR887D APPLICATION NOTE
Rev.4.0
6. Operational Descriptions
6.1 PWM (Pulse Width Modulation) Output Control
The NR887D consists 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.8). 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.
VIN
2
OSC
Σ
5
C2
C1
IN
Current
Sense
1
6
Drive
REG
OCP
BS
M1
PWM
LOGIC
C10
4
M2
R3
3
L1
C4
SW
D1
8
0.8V
C5
R5
R4
Compensation
Error Amp.
VO
5
FB
R6
fig.8 Basic Structure of Chopper Type Regulator with PWM Control by Current Control
The NR887D starts 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 C10 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 the NR887D, 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,
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.11
NR887D APPLICATION NOTE
Rev.4.0
The NR887D integrates 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.9 OCP Characteristics of NR887D
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.10 TSD Characteristics of NR887D
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.4 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.11 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.12 Soft-Start operating description
Copy Right: SANKEN ELECTRIC CO., LTD.
t_delay
t_ss
t_all
Page.12
0.1
NR887D 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.13 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.14).
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 fig15.It starts when a VIN voltage is inputted.
2. IN
100kΩ
7.EN
NR887D
fig. 14 ON / OFF Control 1
2. IN
100kΩ
7.EN
NR887D
fig. 15 ON / OFF Control 2
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.13
NR887D 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.16 shows 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
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
------ (1)
Output Voltage Vo [V]
ILp 
IL
 Io
2
fig.16 Selection Range of Inductance L in NR887D
------ (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.17 Relationship between inductance and ripple current
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.14
NR887D 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. 18 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.19 C1 (C2) Current Waveform
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.15
NR887D 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.
IL
The ripple current effective value of output capacitor is
calculated from the equation (4):
Irms 
IL
2 3
Vo
L1
------ (4)
ESR
When ΔIL = 0.5A,
Irms 
0.5
≒ 0.14A
2 3
IL
fig. 20 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.
Copy Right: SANKEN ELECTRIC CO., LTD.
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. 21 C4 (C5) Current Waveform
NR887D 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 0.5mA is recommended.
(The target of IFB lower limit is 0.5mA, 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
R4+R5
R6
fig. 22 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 NR887D 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.23). Alternatively an external voltage source can be connected
through a diode to the BS Pin (fig.24).
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
NR887D
NR887D
3.SW
3.SW
fig.23 Bootstrap Diode Connection 1
Copy Right: SANKEN ELECTRIC CO., LTD.
fig.24 Bootstrap Diode Connection 2
Page.17
NR887D 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 shows 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 NR887D (fSW =500kHz) VO and C4 (C5) Comparison
C4 (C5) (µF)
VO(V)
Electrolytic Capacitor
Ceramic Capacitor
(ESR≒100mΩ)
1.2
22 to 100
47 to 330
1.8
10 to 68
33 to 220
3.3
6.8 to 68
10 to 100
5
4.7 to 47
6.8 to 100
9
2.2 to 33
3.3 to 33
12
2.2 to 22
2.2 to 22
14
2.2 to 22
2.2 to 22
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.18
NR887D 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.
R3
VIN
2
7
EN
C2
C10
8
SS
C9
VO
L1
SW
3
NR885K
NR887D
R4
C1
1
BS
IN
NC
FB
GND
6
4
5
R4+R5
VFB
C4 C5
R6
D1
Option
IADJ
GND
GND
fig.25 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 inC1,C2
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. Since the
high current is discharged and charged with high speed
through the leads of input / output capacitors, make the
current paths as short as possible. A similar care should be
taken when designing pattern for other capacitors.
C1,C2
fig. 26 Recommended Pattern example
C1,C2
fig. 27 No good pattern example
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.19
C1,C2
NR887D APPLICATION NOTE
Rev.4.0
7.2.3 PCB Layout
R4
EN
R1
SS
C9
BS
C1
R5
NC FB
8
7
6
5
1
2
3
4
R6
GND
R7
C2
R3
C10
IN
D1
C7
C4
L1
SW
NOTES:
Drawing is not to scale.
fig.28 Recommended PCB Layout
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.20
C5
Vo
NR887D 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 fig29.It is tendency that Spike noise becomes
small by reducing the switching-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.
7.3.2 Spike Noise Reduction(2)
・The addition of the Snubber circuit
1.BS
C10
NR887D
fig.29 The addition of the BS serial resistor
2.IN
3.SW
NR887D
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 C7 and R7.
4.GND
≒10Ω
*Option
≒1000pF
fig.30 The addition of the Snubber circuit
7.3.3 Attention about the insertion of the bead-core
fig.31
In the area surrounded by the red dotted line within the fig31, 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
resistor".
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.21
NR887D APPLICATION NOTE
Rev.4.0
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
NR887D
fig.32 Reverse bias protection diode
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.22
NR887D 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 devices or design your products by using our semiconductor devices, 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 devices. 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 (Tj) 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.
 Sanken assumes no responsibility for any troubles, such as dropping products caused during transportation out of
Sanken’s distribution network.
The contents in this document must not be transcribed or copied without Sanken’s written consent.
Copy Right: SANKEN ELECTRIC CO., LTD.
Page.23