Note: Pin numbers are for 8-pin DIP package.
FIGURE 16. 1.2V to 55V Adjustable 500 mA Power Supply with Low Output Ripple
Definition of Terms
A switching regulator topology in which a higher voltage is
converted to a lower voltage. Also known as a step-down
A switching regulator topology in which a positive voltage is
converted to a negative voltage without a transformer.
DUTY CYCLE (D)
Ratio of the output switch’s on-time to the oscillator period.
CATCH DIODE OR CURRENT STEERING DIODE
The diode which provides a return path for the load current
when the LM2574 switch is OFF.
EFFICIENCY ( )
The proportion of input power actually delivered to the load.
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)
The purely resistive component of a real capacitor’s imped-
ance (see Figure 17 ). It causes power loss resulting in ca-
pacitor heating, which directly affects the capacitor’s operat-
ing lifetime. When used as a switching regulator output filter,
higher ESR values result in higher output ripple voltages.
FIGURE 17. Simple Model of a Real Capacitor
Most standard aluminum electrolytic capacitors in the
100 µF–1000 µF range have 0.5
grade capacitors (“low-ESR”, “high-frequency”, or “low-
inductance”) in the 100 µF–1000 µF range generally have
ESR of less than 0.15 .
EQUIVALENT SERIES INDUCTANCE (ESL)
The pure inductance component of a capacitor (see Figure
17 ). The amount of inductance is determined to a large ex-
tent on the capacitor’s construction. In a buck regulator, this
unwanted inductance causes voltage spikes to appear on
OUTPUT RIPPLE VOLTAGE
The AC component of the switching regulator’s output volt-
age. It is usually dominated by the output capacitor’s ESR
multiplied by the inductor’s ripple current ( I
to-peak value of this sawtooth ripple current can be deter-
mined by reading the Inductor Ripple Current section of the
CAPACITOR RIPPLE CURRENT
RMS value of the maximum allowable alternating current at
which a capacitor can be operated continuously at a speci-
STANDBY QUIESCENT CURRENT (I
Supply current required by the LM2574 when in the standby
mode (ON/OFF pin is driven to TTL-high voltage, thus turn-
ing the output switch OFF).
INDUCTOR RIPPLE CURRENT ( I
The peak-to-peak value of the inductor current waveform,
typically a sawtooth waveform when the regulator is operat-
ing in the continuous mode (vs. discontinuous mode).
CONTINUOUS/DISCONTINUOUS MODE OPERATION
Relates to the inductor current. In the continuous mode, the
inductor current is always flowing and never drops to zero,
vs. the discontinuous mode, where the inductor current
drops to zero for a period of time in the normal switching
). The peak-