LM2717_08 NSC [National Semiconductor], LM2717_08 Datasheet - Page 9

• Manufacturer Part Number: LM2717_08
• Description: Dual Step-Down DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

Buck Operation
PROTECTION (BOTH REGULATORS)
The LM2717 has dedicated protection circuitry running during
normal operation to protect the IC. The Thermal Shutdown
circuitry turns off the power devices when the die temperature
reaches excessive levels. The UVP comparator protects the
power devices during supply power startup and shutdown to
prevent operation at voltages less than the minimum 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 also features a shut-
down mode for each converter decreasing the supply current
to approximately 10µA (both in shutdown mode).
CONTINUOUS CONDUCTION MODE
The LM2717 contains current-mode, PWM buck regulators.
A buck regulator steps the input voltage down to a lower out-
put voltage. In continuous conduction mode (when the induc-
tor 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
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.
DESIGN PROCEDURE
This section presents guidelines for selecting external com-
ponents.
SETTING THE OUTPUT VOLTAGE (ADJUSTABLE
REGULATOR)
The output voltage is set using the feedback pin and a resistor
divider connected to the output as shown in Figure 1. The
feedback pin voltage is 1.26V, so the ratio of the feedback
resistors sets the output voltage according to the following
equation:
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is need-
ed betwen the input pin and power ground. This capacitor
prevents large voltage transients from appearing at the input.
The capacitor is selected based on the RMS current and volt-
age requirements. The RMS current is given by:
IN
and SW1 and SW2.
OUT
and the rising current
′
will be re-
9
The RMS current reaches its maximum (I
V
regulators and added to give a total RMS current rating. For
an aluminum or ceramic capacitor, the voltage rating should
be at least 25% higher than the maximum input voltage. If a
tantalum capacitor is used, the voltage rating required is
about twice the maximum input voltage. The tantalum capac-
itor should be surge current tested by the manufacturer to
prevent being shorted by the inrush current. The minimum
capacitor value should be 47µF for lower output load current
applications and less dynamic (quickly changing) load condi-
tions. For higher output current applications or dynamic load
conditions a 68µF to 100µF low ESR capacitor is recom-
mended. It is also recommended to put a small ceramic
capacitor (0.1µF to 4.7µF) between the input pins and ground
to reduce high frequency spikes.
INDUCTOR SELECTION
The most critical parameters for the inductor are the induc-
tance, peak current and the DC resistance. The inductance is
related to the peak-to-peak inductor ripple current, the input
and the output voltages (for 300kHz operation):
A higher value of ripple current reduces inductance, but in-
creases the conductance loss, core loss, and current stress
for the inductor and switch devices. It also requires a bigger
output capacitor for the same output voltage ripple require-
ment. A reasonable value is setting the ripple current to be
30% of the DC output current. Since the ripple current in-
creases with the input voltage, the maximum input voltage is
always used to determine the inductance. The DC resistance
of the inductor is a key parameter for the efficiency. Lower DC
resistance is available with a bigger winding area. A good
tradeoff between the efficiency and the core size is letting the
inductor copper loss equal 2% of the output power.
OUTPUT CAPACITOR
The selection of C
output voltage ripple. The output ripple in the constant fre-
quency, PWM mode is approximated by:
The ESR term usually plays the dominant role in determining
the voltage ripple. Low ESR ceramic, aluminum electrolytic,
or tantalum capacitors (such as Taiyo Yuden MLCC, Nichicon
PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS,
and CDE polymer aluminum) is recommended. An electrolytic
capacitor is not recommended for temperatures below −25°C
since its ESR rises dramatically at cold temperature. Ceramic
or tantalum capacitors have much better ESR specifications
at cold temperature and is preferred for low temperature ap-
plications.
BOOTSTRAP CAPACITOR
A 4.7nF ceramic capacitor or larger is recommended for the
bootstrap capacitor. For applications where the input voltage
is less than twice the output voltage a larger capacitor is rec-
ommended, generally 0.1µF to 1µF to ensure plenty of gate
drive for the internal switches and a consistently low R
(ON)
IN
.
equals 2V
OUT
. This value should be calculated for both
OUT
is driven by the maximum allowable
OUT
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/2) when
DS