LM3578A MWC National Semiconductor, LM3578A MWC Datasheet - Page 8

• Manufacturer Part Number: LM3578A MWC
• Description: Switching Regulator
• Manufacturer: National Semiconductor

www.national.com
Applications Information
C.L. SENSE VOLTAGE MULTIPLICATION
When a larger sense resistor value is desired, the voltage di-
vider network, consisting of R1 and R2, may be used. This
effectively multiplies the sense voltage by (1 + R1/R2). Also,
R1 can be replaced by a diode to increase current limit
sense voltage to about 800 mV (diode V
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout is accomplished with few external
components. When V
breakdown voltage, the output transistor is turned off. This
occurs because diode D1 will then become forward biased,
allowing resistor R3 to sink a greater current from the
FIGURE 6. Current Limit Sense Voltage Multiplication,
FIGURE 7. Current Limit Sense Voltage Multiplication,
FIGURE 5. Current Limit Transient Suppressor,
Ground Referred
V
V
in
in
in
becomes lower than the zener
Referred
Referred
f
+ 110 mV).
DS008711-19
(Continued)
DS008711-20
DS008711-18
8
non-inverting input than is sunk by the parallel combination
of R1 and R2 at the inverting terminal. R3 should be one-fifth
of the value of R1 and R2 in parallel.
MAXIMUM DUTY CYCLE LIMITING
The maximum duty cycle can be externally limited by adjust-
ing the charge to discharge ratio of the oscillator capacitor
with a single external resistor. Typical values are 50 µA for
the charge current, 450 µA for the discharge current, and a
voltage swing from 200 mV to 750 mV. Therefore, R1 is se-
lected for the desired charging and discharging slopes and
C1 is readjusted to set the oscillator frequency.
DUTY CYCLE ADJUSTMENT
When manual or mechanical selection of the output transis-
tor’s duty cycle is needed, the cirucit shown below may be
used. The output will turn on with the beginning of each os-
cillator cycle and turn off when the current sunk by R2 and
R3 from the non-inverting terminal becomes greater than the
current sunk from the inverting terminal.
With the resistor values as shown, R3 can be used to adjust
the duty cycle from 0% to 90%.
When the sum of R2 and R3 is twice the value of R1, the
duty cycle will be about 50%. C1 may be a large electrolytic
capacitor to lower the oscillator frequency below 1 Hz.
FIGURE 9. Maximum Duty Cycle Limiting
FIGURE 8. Under-Voltage Lockout
DS008711-21
DS008711-22