LM3687 Datasheet, PDF (2/35 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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Two enable pins allow the separate operation of either the DC-DC or the Linear Regulator alone or both. If the
power input voltage for the Linear Regulator V_{IN_LIN} is not sufficiently high (e.g. the DC-DC converter is not
enabled or starting up) a startup LDO supplies the Linear Regulator Output from V_BATT for 50mA rated load
current (Startup Mode). If V_{IN_LIN} is at the required voltage level, the startup LDO is deactivated and the main
regulator provides 350mA output current. In shutdown mode (Enable pins pulled low) the device turns off and
reduces battery consumption to 0.1µA (typ.).
The LM3687 is available in a tiny, lead-free (NO PB) 9-bump micro SMD package. A high switching frequency of
1.8MHz (typ.) allows the use of tiny surface-mount components. Only four external components -one inductor
and three ceramic capacitors- are required.
Typical Application Circuit
EN_DCDC
B2
LM3687
EN_LIN
C3
A2
SGND
V_BATT
C1
FB_DCDC
C2
SW
B1
2.2 PH
1.8V
2.7...5.5V
4.7 PF
PGND
A1
10 PF
Load
DC-DC
0..750 mA
Li-Ion
or
3 cell
NiMH/
NiCd
V_{IN_LIN}
B3
V_{OUT_LIN}
A3
2.2 PF
1.5V
Load
LIN
0..350 mA
Figure 1. Typical Application Circuit: Linear Regulator as Post Regulator
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