LM2717-ADJ_08 NSC [National Semiconductor], LM2717-ADJ_08 Datasheet - Page 9

• Manufacturer Part Number: LM2717-ADJ_08
• Description: Dual Step-Down DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

Buck Operation
PROTECTION (BOTH REGULATORS)
The LM2717-ADJ has dedicated protection circuitry running
during normal operation to protect the IC. The Thermal Shut-
down circuitry turns off the power devices when the die tem-
perature reaches excessive levels. The UVP comparator
protects the power devices during supply power startup and
shutdown to prevent operation at voltages less than the min-
imum input voltage. The OVP comparator is used to prevent
the output voltage from rising at no loads allowing full PWM
operation over all load conditions. The LM2717-ADJ also fea-
tures a shutdown mode for each converter decreasing the
supply current to approximately 10µA (both in shutdown
mode).
CONTINUOUS CONDUCTION MODE
The LM2717-ADJ contains current-mode, PWM buck regula-
tors. A buck regulator steps the input voltage down to a lower
output voltage. In continuous conduction mode (when the in-
ductor current never reaches zero at steady state), the buck
regulator operates in two cycles. The power switch is con-
nected between V
In the first cycle of operation the transistor is closed and the
diode is reverse biased. Energy is collected in the inductor
Buck 1 Switch ON Resistance vs. Temperature
Switching Frequency vs. R
IN
and SW1 and SW2.
F
Resistance
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and the load current is supplied by C
through the inductor.
During the second cycle the transistor is open and the diode
is forward biased due to the fact that the inductor current can-
not instantaneously change direction. The energy stored in
the inductor is transferred to the load and output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
where D is the duty cycle of the switch, D and D
quired for design calculations.
The LM2717-ADJ has a minimum switch ON time which cor-
responds to a minimum duty cycle of approximately 10% at
600kHz operation and approximately 5% at 300kHz opera-
tion. In the case of some high voltage differential applications
(low duty cycle operation) this minimum duty cycle may be
exceeded causing the feedback pin over-voltage protection
to trip as the output voltage rises. This will put the device into
a PFM type operation which can cause an unpredictable fre-
quency spectrum and may cause the average output voltage
to rise slightly. If this is a concern the switching frequency may
Buck 2 Switch ON Resistance vs. Temperature
OUT
and the rising current
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will be re-