RC4190
PRODUCT SPECIFICATION
2N2907
R6
Lx
V
OUT
D1
1N914
Z1
R2
R7
C
F
5
R1
V
BAT
R4
6
1
I
C
+V
S
4190
LBR
GND
3
C
X
2
4
L
X
V
FB
7
R3
Cx
R4 =
V
S
- 1.31V
5
µA
260K
50
~
R6 ~
I
L
65-2677
R5
R5 =
10 V
S
R7 ~
~
I
L
Figure 12. Step-Down Regulator Greater Than 30V
Design Equations
The inductor value and timing capacitor (C
X
) value must be
carefully tailored to the input voltage, input voltage range,
output voltage, and load current requirements of the applica-
tion. The key to the problem is to select the correct inductor
value for a given oscillator frequency, such that the inductor
current rises to a high enough peak value (I
MAX
) to meet the
average load current drain. The selection of this inductor
value must take into account the variation of oscillator
frequency from unit to unit and the drift of frequency over
temperature. Use
±20%
as a maximum change from the
nominal oscillator frequency.
The worst-case conditions for calculating ability to supply
load current are found at the minimum supply voltage; use
+V
S
(min) to calculate the inductor value. Worst-case condi-
tions for ripple are at +V
S
(max).
The value of the timing capacitor is set according to the
following equation:
2.4
×
10
-
f
O
(
Hz
)
= ---------------------
C
X
(
pF
)
6
Find a value for the start-up resistor R1:
V
S
– 1.2V
-
R1 = ------------------------
5µA
Find a value for the feedback resistors R2 and R3:
V
OUT
– 1.31V
R2 = -----------------------------------
-
I
A
1.31V
-
R3 = --------------
I
A
Where I
A
is the feedback divider current (recommended
value is between 50
µA
and 100
µA).
Step-Up Design Procedure
1.
2.
Select an operating frequency and timing capacitor as
shown above (10 kHz to 40kHz is typical).
Find the maximum on time (add 5
µS
for the turn-off
base recombination delay of Q1):
1
T
ON
= --------- + 5µs
-
2F
O
The squarewave output of the oscillator is internal and
cannot be directly measured, but is equal in frequency to the
triangle waveform measurable at pin 4. The switch transistor
is normally on when the triangle waveform is ramping up
and off when ramping down. Capacitor selection depends on
the application; higher operating frequencies will reduce the
output voltage ripple and will allow the use of an inductor
with a physically smaller inductor core, but excessively high
frequencies will reduce load driving capability and effi-
ciency.
3.
Calculate the peak inductor current I
MAX
(if this value
is greater than 375 mA, then an external power transis-
tor must be used in place of Q1):
V
OUT
+ V
D
– V
S
-
I
MAX
=
----------------------------------------------------
2I
L
(
F
O
)T
ON
[
V
S
– V
SW
]
where: V
S
= supply voltage
V
D
= diode forward voltage
I
L
= dc load current
V
SW
= saturation voltage of Q1 (typ 0.5V)
9
RAYTHEON [ RAYTHEON COMPANY ]
