LM3687 Datasheet, PDF (22/35 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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V_{OUT_LIN}
V_{OUT_DCDC}
= V_{IN_LIN}
V_{OUT_LIN} + 200 mV
Mode
Switch
(internal)
Switch
Startup
LDO to
Main LDO
Startup LDO
(50 mA)
t_{STARTUP_LIN}
t_{STARTUP_DCDC}
Main LDO
(350 mA)
Figure 9. Startup Sequence, V_{EN_DCDC} = V_{EN_LIN} = V_BATT
Current Limiting
The LM3687 incorporates also a current limit feature for the linear regulator to protect itself and external
components during overload conditions at V_{OUT_LIN}. In the event of a peak over-current condition at V_{OUT_LIN} the
output current through the NFET pass device will be limited.
Application Hints
INDUCTOR SELECTION
There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor
current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current
rating specifications are followed by different manufacturers so attention must be given to details. Saturation
current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of
application should be requested from the manufacturer. The minimum value of inductance to guarantee good
performance is 1.76µH at I_LIM (typ) dc current over the ambient temperature range. Shielded inductors
radiate less noise and should be preferred. There are two methods to choose the inductor saturation current
rating.
Method 1
The saturation current should be greater than the sum of the maximum load current and the worst case average
to peak inductor current. This can be written as:
I_SAT > I_{OUT_DCDC_MAX} + I_RIPPLE
where
I_SAT ! I_OUTMAX + I_RIPPLE
where
I_RIPPLE
=
¨©§^VBAT₂^T
-
x
VL_{OUT¸¹·}
x
¨©§_VVBOAUTTT¸¹·
x
¨©§
1
f
¸¹·
(1)
• I_RIPPLE: average to peak inductor current
• I_{OUT_DCDCMAX}: maximum load current (750mA)
• V_BATT: maximum input voltage in application
• L: minimum inductor value including worst case tolerances (30% drop can be considered for method 1)
• f: minimum switching frequency (1.3MHz)
22
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