NR885E Datasheet, PDF (7/12 Pages) Sanken electric – Synchronous Buck Regulator

3A, 18V, 350kHz , Synchronous Buck Regulator
NR885E
Oct./30/2013
Typical Application Circuit
V_IN
R3
V_IN_s
C1
C2
GND
2
R1
IN
7
EN
C10
1
BS
3
L1
SW
NR885E
R5
8
SS
5
FB
GND
NC
R4
4
6
C9
R6
D1
C1, C2: 10μF / 25V
C4, C5: 22μF / 16V
C9: 0.1μF
C10: 0.1μF
SW
R1: 100kΩ
L1: 10μH
R3: 22Ω
R4: 8.2 kΩ, R5: 4.3kΩ (Vo=3.3V)
R6: 3.9kΩ
Vo
Vo_s
C4 C5
GND
External Components Design Guide
(1)Diode D1
・Flywheel diode D1 can be connected to enhance the efficiency.
・The schottky-barrier diode must be used for D1. If other diodes like fast recovery diodes are used, IC may be
damaged because of the reverse voltage applied by the recovery voltage or ON voltage.
(2)Choke coil L1
・If the winding resistance of the choke coil is too high, the efficiency may go down to the extent that it is out of the rating.
・As the start current of the over current protection is approximately 4A, attention must be paid to the heating of the choke
coil by the magnetic saturation due to overload or short-circulated load.
(3)Capacitor C1( C2), C4(C5), C9
・As large ripple currents across C1 (C2) and C4 (C5), capacitors with high frequency and low impedance for SMPS must
be used. Especially when the impedance of C4 (C5) is high, the switching waveform may not be normal at low
temperature.
・C9 is a capacitor for soft start. In case soft start function is not used, please keep Pin No.2 open.
(4)Resistor R4, R5, R6
・R4, R5, R6 are resistor to the Output Voltage. I_ADJ is required to set to 0.2mA.
R4, R5, R6 are calculated by the equation (1).
R4  R5  ^{VO  VFB }  ^{VO  0.8} ， R6  ^VFB  ^0.8 ≒ 4.0k
I _ADJ
0.2 10 ^3
I _{ADJ 0.210} 3
------- (1)
In order to have optimum operating condition, each component must be connected with the minimum distance.
Rev.1.1
SANKEN ELECTRIC CO., LTD.
7
http://www.sanken-ele.co.jp