Datasheet

LM3208
www.ti.com SNVS404B –APRIL 2006–REVISED MARCH 2013
If V
CON
is over 1.44V (V
OUT
= 3.6V), sub-harmonic oscillation may occur because of insufficient slope
compensation. If V
CON
voltage is less than 0.32V (V
OUT
= 0.8V), the output voltage may not be regulated due to
the required on-time being less than the minimum on-time (55ns). The output voltage can go lower than 0.8V
providing a limited V
IN
range is used. Refer to datasheet curve (V
CON
Voltage vs Output Voltage) for details. This
curve is for a typical part and there could be part-to-part variation for output voltages less than 0.8V over the
limited V
IN
range. When the control pin voltage is more than 0.15V (typ.), the switches are turned on. When it is
less than 0.125V (typ.), the switches are turned off. This on/off function has 25mV (typ.) hysteresis. The
quiescent current when (V
CON
= 0V and V
EN
= Hi) is around 600µA.
Estimation of Maximum Output Current Capability
Referring to Figure 35, the Inductor peak to peak ripple current can be estimated by:
I
IND_PP
= (V
IN
– V
OUT
) × V
OUT
/ (L1 × F
SW
× V
IN
)
where
• Fsw is switching frequency (2)
Therefore, maximum output current can be calculated by:
I
OUT_MAX
= I
LIM
– 0.5 × I
IND_PP
(3)
For the worst case calculation, the following parameters should be used:
F
SW
(Lowest switching frequency): 1.8MHz
I
LIM
(Lowest current limit value): 985mA
L1 (Lowest inductor value): refer to inductor data-sheet. Note that inductance will drop with DC bias current and
temperature. The worst case is typically at 85°C.
For example, V
IN
= 4.2V, V
OUT
= 3.2V, L1 = 2.0µH (Inductance value at 985mA DC bias current and 85°C), F
SW
=
1.8MHz , I
LIM
= 985mA.
I
IND_PP
= 212mA (4)
I
OUT_MAX
= 985 – 106 = 876mA (5)
The effects of switch, inductor resistance and dead time are ignored. In real application, the ripple current would
be 10% to 15% higher than ideal case. This should be taken into account when calculating maximum output
current. Special attention needs to be paid that a delta between maximum output current capability and the
current limit is necessary to satisfy transient response requirements. In practice, transient response requirements
may not be met for output current greater than 650mA.
Inductor Selection
A 3.3µH inductor with saturation current rating over 1200mA and low inductance drop at the full DC bias
condition is recommended for almost all applications. The inductor’s DC resistance should be less than 0.2Ω for
good efficiency. For low dropout voltage, lower DCR inductors are recommended. The lower limit of acceptable
inductance is 1.7µH at 1200mA over the operating temperature range. Full attention should be paid to this limit,
because some small inductors show large inductance drops at high DC bias. These cannot be used with the
LM3208. FDK MIPW3226D3R0M is an example of an inductor with the lowest acceptable limit (as of Oct./05).
Table 1 suggests some inductors and suppliers.
Table 1. Suggested Inductors And Their Suppliers
Model Size (WxLxH) [mm] Vendor
MIPW3226D3R0M 3.2 x 2.6 x 1.0 FDK
1098AS-3R3M 3.0 x 2.8 x 1.2 TOKO
NR3015T3R3M 3.0 x 3.0 x 1.5 Taiyo-Yuden
1098AS-2R7M 3.0 x 2.8 x 1.2 TOKO
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