LM3687 Datasheet, PDF (27/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
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SNVS473A – DECEMBER 2007 – REVISED JULY 2008
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 the Electrical Characteristics section under Enable
Pins (EN_DCDC, EN_LIN), V_IL and V_IH.
FAST TURN ON
For V_{OUT_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 C_OUT of 2.2µF;
assuming V_{IN_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 the section on power dissipation for calculations.
THERMAL-OVERLOAD PROTECTION
Thermal-Overload Protection limits the total power dissipation in the LM3687. When the junction temperature
exceeds T_J = 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 T_J, 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 T_J = +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
V_BATT 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, V_{OUT_LIN} must not exceed V_BATT.
EVALUATION BOARDS
For availability of evaluation boards please refer to the Product Folder of LM3687 at www.national.com. For
information regarding evaluation boards, please refer to Application Note: AN-1647.
Micro SMD PACKAGE ASSEMBLY AND USE
Use of the micro SMD package requires specialized board layout, precision mounting and careful re-flow
techniques, as detailed in National Semiconductor Application Note 1112. Refer to the section "Surface Mount
Technology (SMD) Assembly Considerations". For best results in assembly, alignment ordinals on the PC board
should be used to facilitate placement of the device. The pad style used with micro SMD package must be the
NSMD (non-solder mask defined) type. This means that the solder-mask opening is larger than the pad size.
This prevents a lip that otherwise forms if the soldermask and pad overlap, from holding the device off the
surface of the board and interfering with mounting. See Application Note 1112 for specific instructions how to do
this. The 9-Bump package used for LM3687 has 300 micron solder balls and requires 275 micron pads for
mounting on the circuit board. The trace to each pad should enter the pad with a 90° entry angle to prevent
debris from being caught in deep corners. Initially, the trace to each pad should not exceed 183 micron, for a
section approximately 183 micron long or longer, as a thermal relief. Then each trace should neck up or down to
its optimal width. The important criteria is symmetry. This ensures the solder bumps on the LM3687 re-flow
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