LM3687TL-1813 NSC [National Semiconductor], LM3687TL-1813 Datasheet - Page 18

• Manufacturer Part Number: LM3687TL-1813
• Description: Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode
• Manufacturer: NSC [National Semiconductor]

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comparing equivalent capacitance and voltage ratings in the
1µF to 4.7µF range.
Another important consideration is that tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a tantalum capac-
itor with an ESR value within the stable range, it would have
to be larger in capacitance (which means bigger and more
POWER DISSIPATION AND DEVICE OPERATION
The permissible power dissipation for any package is a mea-
sure of the capability of the device to pass heat from the power
source, the junctions of the IC, to the ultimate heat sink, the
ambient environment. Thus the power dissipation is depen-
dent on the ambient temperature and the thermal resistance
across the various interfaces between the die and ambient
air.
As stated in (Note 6) in the electrical specification section, the
allowable power dissipation for the device in a given package
can be calculated using the equation:
P
For the LM3687 there are two different main sources con-
tributing to the systems power dissipation (P
DC converter (P
Neglecting switching losses and quiescent currents these two
main contributors can be estimated by the following equa-
tions:
• P
• P
with duty cycle D = V
As an example, assuming the typical post regulation applica-
tion, the conversion from V
and further to V
sults in following power dissipations:
P
3.6V)) = 177mW and
P
P
With a θ
P
ΔT
For the same conditions but the linear regulator biased from
V
(because I
T
As lower total power dissipation translates to higher efficiency
this example highlights the advantage of the post regulation
setup.
NO-LOAD STABILITY
Both outputs of the LM3687 will remain stable and in regula-
tion with no external load. This is an important consideration
Capacitance / µF
J
D_SYS
D_DCDC
D_LIN
D_SYS
D_SYS
BATT
of 55K.
D_LIN
D_DCDC
J
= P
, this results in a P
= (1.8V - 1.5V) * 0.35A = 105mW.
10.0
= (T
= 282mW.
will cause a rise of the junction temperature T
4.7
2.2
1.0
D_SYS
= (V
= (0.75A)
JA
= I
OUT_DCDC
J(MAX)
IN_LIN
= 70°C/W for the micro SMD 9 package this
OUT_DCDC
* θ
OUT_LIN
JA
D_DCDC
- T
2
- V
= 20K.
* (0.38Ω * 1.8V / 3.6V + 0.25Ω * (1 - 1.8V /
A
= 400mA) and therefore an increase of
OUT_LIN
OUT_DCDC
) / θ
2
= 1.5V, at maximum load currents, re-
* [(R
) and the linear regulator (P
C1608X5R1A475K
C1608X5R1A225K
C1005X5R1A105K
C1608X5R0J106K
D_LIN
JA
BATT
) * I
DSON(P)
Model
of 735mW, P
/ V
OUT_LIN
= 3.6V to V
BATT
* D) + (R
TABLE 2. Suggested Capacitors and their Suppliers
.
OUT_DCDC
D_DCDC
DSON(N)
D_SYS
Voltage Rating
): the DC-
= 50mW
* (1-D))]
J
= 1.8V
D_LIN
of:
6.3V
10V
10V
10V
).
18
costly) than a ceramic capacitor with the same ESR value. It
should also be noted that the ESR of a typical tantalum will
increase about 2:1 as the temperature goes from 25°C down
to -40°C, so some guard band must be allowed. For the output
capacitor of the DC-DC converter, please note that the output
voltage ripple is dependent on the ESR of the output capaci-
tor.
in some circuits, for example CMOS RAM keep-alive appli-
cations.
ENABLE OPERATION
The outputs of LM3687 may be switched ON or OFF by a logic
input at the Enable pins, V
(related to V
information on startup sequence please refer to 'Operation
Description').
When both enable pins are low, the outputs are off (pins SW
and V
consumes 0.1µA.
If the application does not require the Enable switching fea-
ture, the enable pins should be tied to V
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 the Electrical
Characteristics section under Enable Pins (EN_DCDC,
EN_LIN), V
FAST TURN ON
For V
chitecture 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
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 av-
erage power dissipation will increase depending on the input
to output voltage differences until the thermal shutdown cir-
cuitry will turn off the MOS transistors.
Please refer to the section on power dissipation for calcula-
tions.
THERMAL-OVERLOAD PROTECTION
Thermal-Overload Protection limits the total power dissipation
in the LM3687. When the junction temperature exceeds T
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 (tem-
perature hysteresis), the output MOS transistors are activated
again. This results in a pulsed output voltage during continu-
ous thermal-overload conditions.
As the DC-DC converter in PFM mode (low load current) does
not contribute significantly to an increase of T
Vendor
OUT_LIN
TDK
TDK
TDK
TDK
OUT_LIN
IL
BATT
and V
fast turn-on is guaranteed by an optimized ar-
are high impedance) and the device typically
) at these pins will turn the outputs on (for
IH
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
.
OUT
Type
of 2.2µF; assuming V
EN_DCDC
and V
Case Size / Inch (mm)
EN_LIN
BATT
J
0603 (1608)
0603 (1608)
0603 (1608)
0402 (1005)
, it is not turned
IN_LIN
. A logic high
to keep the
is settled
J
=