LM3208

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SNVS404B – APRIL 2006 – REVISED MARCH 2013

the required on-time being less than the minimum on-time (55ns). The output voltage can go lower than 0.8V

curve is for a typical part and there could be part-to-part variation for output voltages less than 0.8V over the

less than 0.125V (typ.), the switches are turned off. This on/off function has 25mV (typ.) hysteresis. The

Estimation of Maximum Output Current Capability

Referring to Figure 35, the Inductor peak to peak ripple current can be estimated by:

where

•

Fsw is switching frequency

(2)

Therefore, maximum output current can be calculated by:

(3)

For the worst case calculation, the following parameters should be used:

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.

(4)

(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 0.2

Ω 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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