PRODUCT SPECIFICATION
RC4190
4. Find an inductance value for L :
X
2. Build the circuit and apply the worst case conditions to
it, i.e., the lowest battery voltage and the highest load
current at the desired output voltage.
VS – VSW
------------------------
LX(Henries) =
TON
IMAX
3. Adjust the inductor value down until the desired output
voltage is achieved, then go a little lower (approxi-
mately 20%) to cover manufacturing tolerances.
5. The inductor chosen must exhibit approximately this
value at a current level equal to I
.
MAX
6. Calculate a value for the output filter capacitor:
VSIMAX
4. Check the output voltage with an oscilloscope for ripply,
at high supply voltages, at voltages as high as are
TON -------------------- + IL
VOUT
expected. Also check for efficiency by monitoring sup-
CF(µF) = -------------------------------------------------
ply and output voltages and currents [eff = (V
)
OUT
VR
(I
OUT
)/(+V )(I ) x 100%$].
SY
S
where V = ripple voltage (peak)
R
5. If the efficiency is poor, go back to (1) and start over.
If the ripple is excessive, then increase the output filter
capacitor value or start over.
Step-Down Design Procedure
1. Select an operating frequency.
Compensation
2. Determine the maximum on time (T ) as in the step-
ON
When large values (>50 kΩ) are used for the voltage setting
resistors, R2 and R3 of Figure 7, stray capacitance at the
up design procedure.
V
input can add a lag to the feedback response, destabiliz-
FB
3. Calculate I
:
MAX
ing the regulator, increasing low frequency ripple, and lower-
ing efficiency. This can often be avoided by minimizing the
stray capacitance at the V node. It can also be remedied by
FB
adding a lead compensation capacitor of 100 pF to 10 nF in
parallel with R2 in Figure 7.
2IL
IMAX = -----------------------------------------------------------------------
VS – VOUT
----------------------------
(FO)(TON
)
+ 1
V
OUT – VD
4. Calculate L :
X
Inductors
VS – VOUT
Efficiency and load regulation will improve if a quality high
Q inductor is used. A ferrite pot core is recommended; the
wind-yourself type with an air gap adjustable by washers or
spacers is very useful for breadboarding prototypes. Care
must be taken to choose a permeable enough core to handle
---------------------------
(TON)
LX
=
IMAX
5. Calculate a value for the output filter capacitor:
(VS – VOUT)IMAX
the magnetic flux produced at I ; if the core saturates,
MAX
TON ---------------------------------------------- + IL
VOUT
then efficiency and output current capability are severely
degraded and excessive current will flow though the switch
transistor. A pot core inductor design section is provided
later in this datasheet.
CF(µF) = ---------------------------------------------------------------------------
VR
Alternate Design Procedure
The design equations above will not work for the certain
input/output voltage ratios, and for these circuits another
method of defining component values must be used. If the
slope of the current discharge waveform is much less than
the slope of the current charging waveform, then the inductor
current will become continuous (never discharging com-
pletely), and the equations will become extremely complex.
So, if the voltage applied across the inductor during the
charge time is greater than during the discharge time, used
the design procedure below. For example, a step-down
circuit with 20V input and 5V output will have approxi-
mately 15V across the inductor when charging, and approxi-
mately 5V when discharging. So in this example, the
inductor current will be continuous and the alternate
procedure will be necessary.
An isolated AC current probe for an oscilloscope (example:
Tektronix P6042) is an excellent tool for saturation prob-
lems; with it the inductor current can be monitored for non-
linearity at the peaks (a sign of saturation).
Low Battery Detector
An open collector signal transistor Q2 with comparator C2
provides the designer with a method of signaling a display or
computer whenever the battery voltage falls below a pro-
grammed level (see Figure 8). This level is determined by the
+1.3V reference level and by the selection of two external
resistors according to the equation:
R4
VTH = VREF ------ + 1
R5
Where V = Threshold Voltage for Detection
TH
1. Select an operating frequency (a value between 10 kHz
and 40 kHz is typical).
10
RAYTHEON [ RAYTHEON COMPANY ]
