RC4190
PRODUCT SPECIFICATION
L
x
VOUT
2N2907
R6
D1
1N914
CF
Z1
R2
R7
5
4
R1
R4
+VS
LX
VFB
6
1
IC
7
4190
VBAT
LBR
GND
CX
2
R3
3
R5
C
x
50
IL
V
S - 1.31V
~
~
R6
R7
R4 =
R5 =
5 µA
65-2677
10 VS
IL
~
~
260K
Figure 12. Step-Down Regulator Greater Than 30V
Find a value for the start-up resistor R1:
Design Equations
The inductor value and timing capacitor (C ) value must be
X
VS – 1.2V
R1 = ------------------------
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
5µA
Find a value for the feedback resistors R2 and R3:
V
OUT – 1.31V
R2 = -----------------------------------
IA
current rises to a high enough peak value (I ) to meet the
MAX
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.
1.31V
R3 = --------------
IA
Where I is the feedback divider current (recommended
A
value is between 50 µA and 100 µA).
The worst-case conditions for calculating ability to supply
load current are found at the minimum supply voltage; use
Step-Up Design Procedure
1. Select an operating frequency and timing capacitor as
shown above (10 kHz to 40kHz is typical).
+V (min) to calculate the inductor value. Worst-case condi-
S
tions for ripple are at +V (max).
S
2. Find the maximum on time (add 5 µS for the turn-off
The value of the timing capacitor is set according to the
following equation:
base recombination delay of Q1):
2.4 × 106
fO(Hz) = ---------------------
CX(pF)
1
TON = --------- + 5µs
2FO
3. Calculate the peak inductor current I
MAX
(if this value
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.
is greater than 375 mA, then an external power transis-
tor must be used in place of Q1):
VOUT + VD – VS
----------------------------------------------------
2IL
IMAX
=
(FO)TON[VS – VSW
]
where: V = supply voltage
S
V
D
= diode forward voltage
I = dc load current
L
V
SW
= saturation voltage of Q1 (typ 0.5V)
9
RAYTHEON [ RAYTHEON COMPANY ]
