
Page 10
January 1999 TOKO, Inc.
TK11830
APPLICATION INFORMATION
COMPONENT REQUIREMENTS
Inductor
DC resistance of the inductor must be less than 5 
. For
optimal performance and efficiency, an inductor with a DC
resistance of less than 1 
 is recommended. The oscillator
frequency is inversely proportional to inductance. The
inductance should be greater than 300 
μ
H to prevent loss
of efficiency at high frequencies.
There is a large peak current (up to I
LPK
 =  300mA) when
the inductor is saturated.
C
FB
, C
REF
, C
IN
, C
OUT
The filtered output ripple is fed back to the feedback pin. To
ensure continuous operation, C
FB
 should be connected
between the feedback pin and ground. If a large voltage is
fed back to the feedback pin, the power transistor switch
drive will be intermittent. This causes a large ripple voltage
since I
LPK
 becomes larger. The value of C
FB 
 is determined
by the value of the output capacitor, C
OUT,
 and the feedback
resistance, R
2
. The feedback capacitor must be larger
when the ripple voltage is high due to the lower C
OUT
. C
REF
is used to prevent oscillation of the band gap reference and
to stabilize the feedback loop. The input capacitor, C
IN
, is
used to reduce supply impedance and to provide sufficient
input current during switching for stable circuit operation.
Recommended values:
C
REF
 > 0.1 
μ
F
C
FB
 > 0.01 
μ
F
C
IN
 > 22 
μ
F
C
OUT
 > 22 
μ
F
Note:  C
OUT
 should be sufficiently large and have a low
ESR to minimize ripple voltage.
Control Pin Resistor (R
CONT
)
Input requirements of the Control pin are as follows:
When V
CONT
 is high (above 2.2 V), the circuit operation is
stopped. When V
CONT
 is low (below 0.4 V), operation is
resumed.
A control current of 3 
μ
A (typ.) is required for shutdown.
Shutdown voltage, V
CONT
, is related to the resistance
R
CONT
 as shown below. V
CONT
 changes when R
CONT
 is
changed.
V
CONT
 ~ R
CONT 
x I
CONT
 + V
BE
V
CONT
 ~ (300 k
) x (3 
μ
A) + 0.7 V = 1.60 V at
R
SD
 = 300 k
 and V
BE
 ~ 0.7 V
ICONT
+
VCONT
RCONT
VBE
30 k
ON/OFF CONTROL
-4
-2
V
-5
-3
-1
VCONT
0
TA = 25 
°
C
 0                            1                            2
I
4
2
5
3
1
0
ILPK
IL
t
L(SMALL)
L(LARGE)
t
ILPK(MAX)
-300 mA
INDUCTOR