LM5007
Hysteretic Control Circuit
Overview
(Continued)
output capacitor, as shown in
The series resistor
(R) will degrade the load regulation. Another technique for
enhancing the ripple voltage at the FB pin is to place a
capacitor in parallel with the feedback divider resistor R1.
The addition of the capacitor reduces the attenuation of the
ripple voltage from the feedback divider
High Voltage Bias Regulator
The output voltage (Vout) can be programmed by two exter-
nal resistors as shown in
The regulation point can
be calculated as follows:
V
OUT
= 2.5 x (R1 + R2) / R2
The feedback comparator in hysteretic regulators depend
upon the output ripple voltage to switch the output transistor
on and off at regular intervals. In order for the internal
comparator to respond quickly to changes in output voltage,
proportional to inductor current, a minimum amount of ca-
pacitor Equivalent Series Resistance (ESR) is required. A
ripple voltage of 25mV to 50mV is recommended at the
feedback pin (FB) for stable operation. In cases where the
intrinsic capacitor ESR is too small, additional series resis-
tance may be added.
For applications where lower output voltage ripple is re-
quired the load can be connected directly to the low ESR
The LM5007 contains an internal high voltage bias regulator.
The input pin (Vin) can be connected directly to line voltages
from 9 to 75 Volts. To avoid supply voltage transients due to
long lead inductances on the input pin (Vin Pin 8), it is always
recommended to connect low ESR ceramic chip capacitor
() 0.1µF) between "Vin" pin and "RTN" pin (pin 4), located
close to LM5007. The regulator is internally current limited to
10mA. Upon power up, the regulator is enabled and sources
current into an external capacitor connected to the Vcc pin.
When the voltage on the Vcc pin reaches the regulation point
of 7V, the controller output is enabled.
An external auxiliary supply voltage can be applied to the
Vcc pin. If the auxiliary voltage is greater than 7 Volts the
internal regulator will essentially shutoff, thus reducing inter-
nal power dissipation.
20078305
FIGURE 1. Low Ripple Output Configuration
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