5.4 Negative Switching Regulator (PWM)
5.4.7 Basic Design Approach
As shown in Fig. 5-23, most of the design values have been worked out. Use these values as a starting point for simplified design. The following is a brief discus
sion concerning the design values.
The RpR2 ratio determines VQUT. The equivalent resistance of this combina
tion should be kept in the range from 25 to 100 kQ to minimize input-bias current and input-current noise errors. The manufacturer recommends RN55 metal-film re
sistors for Rj and R2.
Resistor R3 sets the shunt-regulator (zener at pin 5) current. This current and the power loss in R3 represent the major sources of efficiency losses in a typical 4292 application. (Figure 5-24 shows an efficiency of 60% with a 7.5-kQ value for R3.) Use a fixed resistance for R3 when the input voltage is fixed. For variable-input applications, a current source such as shown in Fig. 5-25 will improve efficiency over a wide range of input voltages.
Other alternatives to a fixed resistor for R3 include putting an extra winding on the transformer T{ and generating the shunt current with a rectifier diode and fil
ter capacitor. If a negative output greater than -15 V is generated, a bootstrap circuit can be made by tapping the output with a diode and resistor connected to pin 5.
2N3906 or equiv.
Figure 5-23. Negative PWM flyback regulator basic connections (Raytheon Linear Integrated Circuits, 1989, p. 9-43)
Typical Specifications for 4292 Flyback Application (Figure 3)
(TA = +25°C. VQUT = +5.0V unless otherwise specified) Parameters
Input Range 1 Input Range
Output Voltage Line Regulation Load Regulation Load Regulation Efficiency Output Ripple
Test Conditions R3 = 5kü R3 = 7.5kil
V,N = -48V, lL = +60mA R3 = 5kil,-60V to-30V|N, lL = 60mA
V,N = -48V, lL = 10mA to 120mA V,N = -48V, lL = 0 to 10mA R3 = 7.5kil, V,N = -48V ILOAD = 120mA V|N = 48V. IL= 120mA
Min -60 -90
Typ -48 -48 5.0 20 15 20 60 40
Max -30 -40
Unite | V V V mV mV mV
%
mVp.p
Figure 5-24. Negative PWM flyback regulator electrical characteristics (Raytheon Lin
ear Integrated Circuits, 1989, p. 9-43)
To Pin 5
I
25K
5.1V
Zener ' 510
► 1/2W
- V I N
Figure 5-25· Current-source for variable input applications (Raytheon Linear Integrated Circuits, 1989, p. 9-44)
The low value (0.5 Ω) for resistor R4 sets the maximum switch current. Cur
rent through switch Mj develops a voltage across R4. This voltage is sensed by the current comparator. The level at which the voltage triggers the current comparator to end the cycle is a function of the differential signal received by the error amplifier, as shown in Fig. 5-26.
The purpose of designing this characteristic into the 4292 is to ensure that each cycle delivers the correct amount of charge to the output. For example, if the output has a light load, the feedback-voltage differential (AyFB) is small with each cycle. Under these conditions, it is desirable to keep each charging pulse small (by cutting the on-portion) to reduce output voltage ripple. The current comparator does this, keeping ripple low over a wide range of load currents.
Resistor R5 holds Mj off during start-up or any time the 4292 is inactive. Re
sistor R6 provides for a signal loss in the gate drive to Mp preventing possible oscil
lation. (R6 is not always required.) Resistor R7 cancels input-bias current errors at the error amplifier input and is also optional.
Resistor R8, diode Dp and capacitor C3 form a "scrubber" network that damp
ens ringing on the M{ drain and the Tj primary. The R8/Dj/C3 network reduces volt-
200
£ 100
I
ï ·
a
Jfc.
-200
-30 -20 -10 0 10 20 30 VFBdMf(mV)
(♦V«) - (-VFB)
Figure 5-26· Feedback voltage versus feedback current (Raytheon Linear Integrated Circuits, 1989, p. 9-36)
age spikes that might potentially overvoltage and damage Mx (depending on the type of field-effect transistor [FET] used for Mx). The network may not be required for all types of FETs, and a small increase in efficiency will be gained by omitting the network.
Capacitor Cx determines the oscillator frequency, as shown by the graph of Fig. 5-27. Silver mica capacitors are recommended because of their good tempera
ture coefficients. Operating frequencies in the range from 60 to 100 kHz are typical.
High frequencies allow the use of a physically small transformer Tr Lower frequen
cies improve efficiency because switching losses are reduced.
Capacitors C4 and C5 filter the shunt-regulator voltage. If the shunt current goes too low to supply the 4292 properly, the IC starts to "motor boat," where the output turns off and on at a low frequency. This frequency varies with the value of C5. Capacitor C4 should have a low impedance to high frequencies as the purpose of C4 is to filter switching noise. Capacitors C6 and C7 are output-filter capacitors.
Diode D2 is the output rectifier. A power Schottky diode, such as the 1N5818 shown, is recommended for best efficiency.
Oscillator Frequency vs. Cx
0 50 100 150 200 250 300 CxInpF
Figure 5-27. Oscillator frequency versus capacitor Cx (Raytheon Linear Integrated Cir
cuits, 1989, p. 9-36)
The fundamental limitation of the maximum load power that can be extracted from the 4292 supply is determined by the gain-to-drain capacitance of external FET M, (a MOSFET). Although specifically designed to drive capacitive loads, the VDRIVE output from pin 6 will not switch large FETs (where drain current exceeds 10 A). The maximum FET size is also affected by the ratio of -VIN to VOUT, because that ratio determines the effective gain of the FET and therefore the Miller capaci
tance. The manufacturer recommends an International Rectifier IRF9633 (1.2-Ω channel resistance, 150-V breakdown, or a Motorola MTP5P18) (1.0-Ω channel re
sistance, 180-V breakdown) for Mr
5.4.2 Negative-Input Negative-Output Regulator
Figure 5-28 shows a negative-input, positive-output regulator (with trans
former) similar to that of Fig. 5-23. In the circuit of Fig. 5-28, the transformer Tj and diode D2 are connected so as to produce a negative-output voltage (diode polar
ity is reversed). The feedback signal is also applied to the -VB B input (pin 2) so as to maintain the correct sense of feedback polarity. All of the design notes in Section 5.4.1 apply to the circuit of Fig. 5-28. In applications where the negative-output voltage is twice the negative-input voltage, a two-terminal inductor can be used in
stead of a transformer, as shown in Fig. 5-29.
5.4.3 Dual-Output PBX Applications
Figure 5-30 shows a regulator nearly identical to the circuit of Fig. 5-23, ex
cept for a center-tapped transformer Tp and additional components to create a nega
tive-output voltage. The component values shown are selected for use with the -48-V off-hook voltage of a branch office or PBX telephone line. All of the design notes in Section 5.4.1 apply to the circuit of Fig. 5-30.
The positive output is normally regulated by the PWM circuits. The negative voltage is unregulated, but will track the positive voltage if the voltage drops on D{
and D2 are matched. This type of regulation, through the magnetic loading produced by secondary taps, is best suited for applications where the load current is relatively constant.
Transformer design is critical for best efficiency and minimum core size. The transformer shown for Tj (AIE Magnetics, Inc.) is designed to deliver +5 and -5.5 V. To get different voltages, or to meet other load requirements, the turns ratio, core size, and core air gap might require adjustment. Use the procedures in Chapter 3 for designing transformers other than as recommended in Fig. 5-30.
5.4.4 Low-Power Switched-Capacitor Regulator
Figure 5-31 shows the 4292 in a circuit that does not require an inductor or transformer. Instead, the circuit uses the VDRIVE output (pin 6) to charge a capacitor up to the shunt voltage and then to switch the more negative terminal to ground.
—à ONG
Γ
C4 0.1MF-T- __ es o— -VINT u
3_ C J-» -
RS 10K R4 0.5(1 ICM, T VOUT V«f t ("*)
01 1M414· J.C3 T1· T1C Figure 5-28· Negative-input, negative-output operation (Raytheon Linear Integrated Circuits, 1989, p. 9-46
ONO
π
C4_ 0.1 MFT- csC «
X V
LOOIC -H-■ΛΛΛ» #
R5 10K * V ~"!¥ «'('--Si) R1//R2 RT^SOK
n
IWT T-M 1— i
i C7 Figure 5-29. Negative-input, negative-output operation with inductor (Raytheon Linear Integrated Circuits, 1989, p. 9-46M1 IRFM33 01.02 1Ν5Π8 03 1N4148 T1 AIE 31S-0M6'
'AIE Mognotic». Inc (615) 244-9024
"To nrtuc· high-tnjquoncy nota· - V I N O-
Figure 5 - 3 0 · Dual-output PBX operation (Raytheon Linear Integrated Circuits, 1989, p. 9-47)
GND
1/2W 1/iW
O β5-034?βΑ -V|N2
Note: Use -V,N2 for V,N = -20V to -35V
Figure 5-31 · Low-power switched-capacitor regulator (Raytheon Linear Integrated Cir
cuits, 1989, p. 9-47)
This is similar to other switched-capacitor regulators such as the ICL7660. The load-current capability is limited. For example, with a +5-V output, the maximum current ranges between 10 and 20 m A.