LM3687 Datasheet, PDF (4/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
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Note:The actual physical placement of the package marking will vary from part to part. The package marking "X"
designates the date code. "T" is a NSC internal code for die traceability. Both will vary considerably. "BC"
identifies the device (part number, option, etc.)
Pin Functions
Pin Number
A1
A2
A3
B1
B2
Pin Name
PGND
SGND
V_{OUT_LIN}
SW
EN_DCDC
B3
V_{IN_LIN}
C1
V_BATT
C2
FB_DCDC
C3
EN_LIN
Pin Descriptions
Description
Power Ground pin
Signal Ground pin
Voltage Output of the linear regulator
Switching Node Connection to the internal PFET switch and NFET synchronous rectifier
Enable Input for the DC-DC converter. The DC-DC converter is in shutdown mode if voltage at this pin is <
0.4V and enabled if > 1.0V. Do not leave this pin floating. Please see section 'Enable Combinations'.
Power Supply Input for the linear regulator
Power Supply for the DC-DC output stage and internal circuitry. Connect to the input filter capacitor (see
typical application).
Feedback Analog Input for the DC-DC converter. Connect directly to the output filter capacitor.
Enable Input for the linear regulator. The linear regulator is in shutdown mode if voltage at this pin is < 0.4V
and enabled if > 1.0V. Do not leave this pin floating. Please see section 'Enable Combinations'.
Voltage Options
DC-DC Converter Output:
V_{OUT_DCDC}
1.80V
1.80V
1.80V *
Linear Regulator Output:
V_{OUT_LIN}
1.50V
1.20V
1.30V *
* For availability of these or other output voltage combinations please contact your local NSC sales office
Enable Combinations
EN_DCDC
0
0
1
1
EN_LIN
0
1
0
1
Comments
No Outputs
Linear Regulator enabled only *
DC-DC converter enabled only
DC-DC converter and linear regulator active *
* Startup Mode:
V_{IN_LIN} must be higher than V_{OUT_LIN(NOM)} + 200mV in order to enable the main regulator (I_MAX = 350mA).
If V_{IN_LIN} < V_{OUT_LIN(NOM)} + 100mV (100mV hysteresis), the startup LDO (I_MAX = 50mA) is active, supplied from
V_BATT.
For example in the typical post regulation application the LDO will remain in startup mode until the DC-DC
converter has ramped up its output voltage.
Order Information
Output Voltage Option
V_{OUT_DCDC}
V_{OUT_LIN}
1.80V
1.50V
Order Number
LM3687TL-1815
LM3687TLX-1815
Package Marking
S9
S9
Supplied as
250 units, tape and reel,
lead free
3000 units, tape and reel,
lead free
Flow
NOPB
NOPB
4
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