where I A is the feedback divider current (recommended value is between 50 and 100 μΑ)
4. Test the circuit for load-line regulation, efficiency, and ripple as described in Chapter If the circuit is within tolerance, quit while you are ahead! If
5.3 Inverting and Step-Down Switching Regulator
Figures 5-12, 5-13, and 5-14 show the basic connections, waveforms, and electrical characteristics for a typical switching regulator (the Raytheon RC4391) connected in the inverting configuration. Figures 5-15 and 5-16 show the 4391 con
nected in the step-down configuration.
The 4391 regulator provides all of the active functions needed to create sup
plies for micropower circuits. The internal circuits consist of an oscillator, switch, reference, comparator, and logic, plus a discharged-battery detector. The regulator can provide up to a typical 70% efficiency in most applications while operating over a supply range of 4 to 30 V at a quiescent current drain of 170 μΑ.
The standard application circuit requires an inductor, diode, two resistors, a low-value timing capacitor, and an electrolytic filter capacitor. The 4391 comple
ments the 4190 (see Section 5.2), which is dedicated to step-up operation. The 4391 is designed for inverting (positive-input, negative-output) and step-down ap
plications.
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Parts List R1- R2- U-
-5.0V Output 300kn 75kn 150pF
-15 Outpu 900k 75ki 150p 1.0mHDaleTE3Q4T -VOUT = (1.25V) (Q) 'Caution: Use current limiting protection circuit for high values of CF Figure 5-12· Inverting regulator basic connections (Raytheon Linear Integrated Circuits, 1989, p. 9-54)
\JU~U
(Internal) }0sc© Φ
•LOAD . - ..— OmA
ΙΓ~ΙΓ~ΙΓ~ vr~[T~ U 3
— V s (Internal) }vB E Q17V
— - 'MAX ' ILX
Figure 5-13. Inverting regulator waveforms (Raytheon Linear Integrated Circuits, 1989, p. 9-55)
VLx
5.3.I Basic Design Approach
The inductor value and timing-capacitor Cx value must be carefully tailored to the input voltage, input voltage range, output voltage, and load-current requirements of the application. (The values shown in Fig. 5-12 are for the inverting configura
tion with outputs of-5 or -15 V.) The key to the problem is to select the correct in
ductor value for a given oscillator frequency, such that the inductor current rises to a high enough peak value (IMAX) to meet the average load-current drain. The selec
tion of the inductor value must take into account the variation of oscillator fre
quency from IC to IC and the drift of frequency over temperature. Use ±30% as a maximum change from the nominal oscillator frequency.
The value of the timing capacitor is set by frequency (fQ in (Hz): (4.1 x l O- 6) / ^ The square-wave output of the oscillator is internal and cannot be directly measured, but it is equal in frequency to the triangle waveform at pin 3. The switch transistor is normally on when the triangle waveform is ramping up, and off when it is ramping down.
Capacitor selection depends on the application. Higher operating frequencies reduce the output-voltage ripple and allow the use of an inductor with a physically smaller size. However, excessively high frequencies reduce load-driving capability and efficiency.
Electrical Characteristics
(Vs = +6.0V, TA = +25° C unless otherwise noted) Parameter
Supply Current
Output Voltage
Line Regulation
Load Regulation
Reference Voltage Switch Current Switch Leakage Current 1 Timing Pin Current 1 LBD Leakage Current 1 LBD on Current 1 LBR Bias Current
Symbol
ISY
VOUT
VREF
•sw
•co 'cx
Condition VS = +4.0V, No External Loads Vs = +25V, No External Loads VoUT nom = -5.0V VoUT nom = "15V VoUT nom = "5.0V, CX = 150pF, Vs = +5.8V to +15V VOUT nom = -15V, Cx = 150pF.
VS = +5.8V to +15V VOUT nom = -5.0V, Cx = 350pF. Vs = +4.5V, PLOAD = OmW to 75mW VOUT nom = -15V, Cx = 350pF, Vs = +4.5V.
PLOAD = OmW to 75mW
Pin 5 = 5.5V Pin 5 = -24V Pin 3 = OV
Pin 1 = 1.5V, Pin 2 = 6.0V Pin 1 = 1.1V, Pin 2 = 0.4V Pin 1 = 1.5V
Min
-5.35 -15.85
1.18 75 6.0 210
2.
Typ 170 300 -5.0 -15 1.5 1.0 0.2
0.07 1.25 100 0.01 10 0.01 600
» 0.7
Max 250 500 -4.65 -14.15
3.0 2.0 0.4
0.14 1.32 5.0 14 5.0
Units |
μΑ
V
%VouT
%VOUT
V mA μΑ μΑ μΑ
♦ μΑ μΑ
Figure 5-14. Step-down/inverting regulator electrical characteristics (Raytheon Linear Integrated Circuits, 1989, p. 9-52)
5.3.2 Inverting Design Approach
Use the following approach for the basic inverting circuit of Fig. 5-12.
1. Select an operating frequency and timing capacitor using the equation of Section 5.3.1. If the output is to be either - 5 V or -15 V, use 150 pF as shown in Fig. 5-12. A frequency of 10 to 50 kHz is typical.
2. Find the maximum on-time (add 3 ps for the turn-off delay of Qx):
l0N ~ 1 2L·
+ 3μ8.
3. Calculate the peak inductor current IMAX (if this value is greater than 375 mA, then an external power transistor must be used in place of Q,):
MAX
(VQUT + VD )2 IL
(f(y *ON ( * s ~ * sw'
J \ 01 1N914
Important Note: This circuit must have a minimum load > 1 mA always connected
Figure 5-15· Step-down regulator basic connections (Raytheon Linear Integrated Cir
cuits, 1989, p. 9-56)
where
Vs = supply voltage
VD = diode forward voltage (typically 0.7 V) IL = DC load current
Vs w = saturation voltage of Qj (typically 0.5 V) 4. Find an inductance value for Lx:
(
s sw\ *MAX / I ON*The inductor chosen must show approximately this value at a current level equal to that of IMAX. If the output is to be either -15 V or -5 V, use 1 mH, as shown in Fig. 5-12.
5.3.3 Step-Down Design Approach
Use the following approach for the basic step-down circuit of Fig. 5-15.
1. Select an operating frequency.
2. Determine the maximum on-time (tON) as in the inverting design proce
dure.
(Internal) Osc
©
-IL \
\ 'LOAD
ΙΓ~ΙΓΊΓ~ i n r ~ i_r:^l
iM VuliT-VfiAI
"/0\
yB AT VOUT - VBAT / * — 'MAX
Î
lLXFigure 5-16. Step-down regulator waveforms (Raytheon Linear Integrated Circuits, 1989, p. 9-57)
3. Calculate IMAX:
I
= 2_L
(fo)(W [
™s *ουτ) . ( ^ουτ " * D) J4. Calculate Lx:
L x
/ v ^ A
t o N\ XMAX /
5.3.4 Alternate Design Procedures
The design equations in Sections 5.3.2 and 5.3.3 will not work for certain input-output voltage ratios. If the inductor current becomes continuous (see Section
1.5.2), the equations become very complex. For example, a step-down circuit with a 20-V input and a 5-V output has about 15 V across the inductor during charge, and about 5 V during discharge. The inductor is never fully discharged at any time. The following alternate procedure, although designed for continuous operation, will also work for the discontinuous mode.
1. Select an operating frequency based on electromagnetic interference (EMI) and component-size requirements (a value between 10 and 50 kHz is typi
cal).
2. Build the circuit, and apply the worst-case conditions (lowest battery volt
age and highest load current at the desired output voltage).
3. Select an inductor value until the desired output voltage is reached, using the equations for Lx from Sections 5.3.2 and 5.3.3 as a guide. For step- down applications, select an inductor that will produce an output voltage slightly less than desired (to allow for manufacturing tolerances). Remem
ber that the actual output voltage is set by the R,:R2 ratio (see Figs. 5-12 through 5-15).
4. Test the circuit for load-line regulation, efficiency, and ripple as described