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LM3687 Datasheet(PDF) 21 Page - Texas Instruments

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Part # LM3687
Description  Step-Down DC-DC Converter
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

LM3687 Datasheet(HTML) 21 Page - Texas Instruments

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LM3687
www.ti.com
SNVS473B – DECEMBER 2007 – REVISED MAY 2013
ENABLE OPERATION
The outputs of LM3687 may be switched ON or OFF by a logic input at the Enable pins, VEN_DCDC and VEN_LIN. A
logic high (related to VBATT) at these pins will turn the outputs on (for information on startup sequence please
refer to Operation Description).
When both enable pins are low, the outputs are off (pins SW and VOUT_LIN are high impedance) and the device
typically consumes 0.1µA.
If the application does not require the Enable switching feature, the enable pins should be tied to VBATT to keep
the outputs permanently on.
To ensure proper operation, the signal source used to drive the enable inputs must be able to swing above and
below the specified turn-on/off voltage thresholds listed in Electrical Characteristics: Enable Pins (EN_DCDC,
EN_LIN), VIL and VIH.
FAST TURN ON
For VOUT_LIN fast turn-on is guaranteed by an optimized architecture allowing a fast ramp of the output voltage to
reach the target voltage while the inrush current is controlled low at 120mA typical (for a COUT of 2.2µF;
assuming VIN_LIN is settled before enable happens).
SHORT-CIRCUIT PROTECTION
Both outputs of the LM3687 are short circuit protected and in the event of a peak over-current condition, the
output current through the MOS transistors will be limited.
If the over-current condition exists for a longer time, the average power dissipation will increase depending on
the input to output voltage differences until the thermal shutdown circuitry will turn off the MOS transistors.
Please refer to POWER DISSIPATION AND DEVICE OPERATION for calculations.
THERMAL-OVERLOAD PROTECTION
Thermal-Overload Protection limits the total power dissipation in the LM3687. When the junction temperature
exceeds TJ = 160°C typ., the shutdown logic is triggered and the output MOS transistors are turned off, allowing
the device to cool down. After the junction temperature dropped by 20°C (temperature hysteresis), the output
MOS transistors are activated again. This results in a pulsed output voltage during continuous thermal-overload
conditions.
As the DC-DC converter in PFM mode (low load current) does not contribute significantly to an increase of TJ, it
is not turned off in case a thermal shutdown is initiated. If the DC-DC converter operates in PWM mode, the
PMOS is turned off in case of a thermal shutdown.
The Thermal-Overload Protection is designed to protect the LM3687 in the event of a fault condition. For normal,
continuous operation, do not exceed the absolute maximum junction temperature rating of TJ = +150°C (see
Absolute Maximum Ratings).
REVERSE CURRENT PATH
There are two body diodes at the switch pin of the DC-DC converter. It is not allowed to pull the switch pin above
VBATT or below PGND by more than 200mV.
On the main linear regulator there is a bulk switching feature in place preventing the parasitic diode structures
from conducting current. This feature is only active as long as any of the regulators is enabled.
For the startup LDO, VOUT_LIN must not exceed VBATT.
EVALUATION BOARDS
For availability of evaluation boards please refer to the Product Folder of LM3687 at www.ti.com. For information
regarding evaluation boards, please refer to Application Note: AN-1647 SNVA249.
Copyright © 2007–2013, Texas Instruments Incorporated
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