lm4510sdx National Semiconductor Corporation, lm4510sdx Datasheet - Page 11

• Manufacturer Part Number: lm4510sdx
• Description: Synchronous Step-up Dc/dc Converter With True Shutdown Isolation
• Manufacturer: National Semiconductor Corporation

Synchronous boost converter is shown in Figure 3. At the start
of each cycle, the oscillator sets the driver logic and turns on
the NMOS power device and turns off the PMOS power de-
vice.
Cycle 1 Description
Refer to Figure 4. NMOS switch turn-on
increases and flows to GND.
PMOS switch turn-off
pacitor supplies load current.
During operation, EAMP output voltage (V
larger loads and decreases for smaller loads. When the sum
of the ramp compensation and the sensed NMOS current
reaches a level determined by the EAMP output voltage, the
PWM COMP resets the logic, turning off the NMOS power
device and turning on the PMOS power device.
Cycle 2 Description
Refer to Figure 5. NMOS Switch turn-off
on
→
load current.
After the switching period the oscillator then sets the driver
logic again repeating the process.
ON/OFF CONTROL
The LM4510 shuts down when the EN pin is low. In this mode
the feedback resistors and the load are disconnected from the
input in order to avoid leakage current flow and to allow the
output voltage to drop to 0V.
The LM4510 turns on when EN is high. There is an internal
pull-down resistor on the EN pin so the device is in a normally
off state.
→
Inductor current recharges output capacitor and supplies
Inductor current decreases and flows through PMOS
FIGURE 4. Equivalent Circuit During Cycle 1
FIGURE 5. Equivalent Circuit During Cycle 2
→
Isolate VOUT from SW
→
COMP
PMOS Switch turn-
→
Inductor current
) increases for
→
Output ca-
30031015
30031016
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SHORT CIRCUIT PROTECTION
When V
switching due to the short-circuit protection circuitry and the
short-circuit output current is limited to I
FEEDBACK FAULT PROTECTION
The LM4510 features unique Feedback Fault Protection to
maximize safety when the feedback resistor is not properly
connected to a circuit or the feedback node is shorted directly
to ground.
Feedback fault triggers V
if V
When the feedback network is reconnected and V
than the OFF threshold level of Feedback Fault Protection,
V
loop.
INPUT UNDER-VOLTAGE LOCK-OUT
The LM4510 has dedicated circuitry to protect the IC and the
external components when the battery voltage is lower than
the preset threshold. This under-voltage lock-out with hys-
teresis prevents malfunctions during startup or abnormal
power off.
THERMAL SHUTDOWN
If the die temperature exceeds 150°C (typ.), the thermal pro-
tection circuitry shuts down the device. The switches remain
off until the die temperature is reduced to approximately
140°C (typ.).
Application Information
ADJUSTING OUTPUT VOLTAGE
The output voltage is set using the feedback pin and a resistor
voltage divider (R
in the Typical Application Circuit.
The ratio of the feedback resistors sets the output voltage.
R
First of all choose a value for R
and 25 kΩ.
R
Calculate R
Table 1 gives suggested component values for several typical
output voltages.
OUT
F2
F1
OUT
Selection
Selection
monitoring stops. V
reaches a protection level, the device shuts down.
OUT
F1
goes down to V
using the following equation:
F1
, R
F2
) connected to the output as shown
OUT
OUT
IN
is then regulated by the control
monitoring. During monitoring,
–0.7V (typ.), the device stops
F2
generally between 10 kΩ
INIT_CHARGE
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OUT
.
is lower