LM2715MTX-ADJ NSC [National Semiconductor], LM2715MTX-ADJ Datasheet - Page 14

• Manufacturer Part Number: LM2715MTX-ADJ
• Description: TFT Panel Module
• Manufacturer: NSC [National Semiconductor]

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Operation
CONTINUOUS CONDUCTION MODE
The LM2715 is a TFT Panel Module containing a current-
mode, PWM boost regulator. A boost regulator steps the
input voltage up to a higher output voltage. In continuous
conduction mode (when the inductor current never reaches
zero at steady state), the boost regulator operates in two
cycles.
In the first cycle of operation, shown in Figure 1 (a), the
transistor is closed and the diode is reverse biased. Energy
is collected in the inductor and the load current is supplied by
C
The second cycle is shown in Figure 1 (b). During this cycle,
the transistor is open and the diode is forward biased. 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' will be
required for design calculations
OUT
.
(a) First Cycle of Operation (b) Second Cycle Of Operation
FIGURE 1. Simplified Boost Converter Diagram
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14
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the feedback pin and a
resistor divider connected to the output as shown in the
typical operating circuit. The feedback pin voltage is 1.265V,
so the ratio of the feedback resistors sets the output voltage
according to the following equation:
SOFT-START
The LM2715 has patented internal circuitry that is used to
limit the inductor inrush current on start-up of the boost
DC/DC switching regulator. This inrush current limiting cir-
cuitry serves as a soft-start. The soft-start time is set by the
internal soft-start circuitry, typically 7ms.
DELAY CAPACITOR
The LM2715 has internal circuitry that can be used to set a
delay time preventing control of the PMOS switch via SWC
until a desired amount of time after the switcher starts up.
The PMOS control circuitry remains inactive until V
reaches 85% of the nominal output voltage. When this oc-
curs, C
pin reaches 1.265V the PMOS switch will become active and
can be controlled using the SWC pin. If no C
PMOS switch can be controlled immediately after V
reaches 85% of the nominal output voltage. The delay time
can be calculated using the equation:
D
begins to charge. When the voltage on the Delay
T
D
= C
D
* (1.265V/5.7µA)
D
is used, the
OUT
OUT