ML4790
efficiency and maximum output current, and if a current
probe is available, look at the inductor current to see if it
looks like the waveform shown in Figure 5.
BOOST CAPACITOR
The boost capacitor (C2) supplies current to the load
during the ON-time of Q1 and will limit the ripple the
LDO stage has to contend with. The ripple on C2 is
influenced by three capacitor parameters: capacitance,
ESL, and ESR. The contribution due to capacitance can be
determined by looking at the change in the capacitor
voltage required to store the energy delivered by the
inductor in a single charge-discharge cycle, as given by
the following formula:
The DC resistance of the inductor should be kept to a
minimum to reduce losses. A good rule of thumb is to
allow 5 to 10mΩ of resistance for each µH of inductance.
Also, be aware that the DC resistance of an inductor
usually isn‘t specified tightly, so an inductor with a
maximum DC resistance spec of 150mΩ may actually
have 100mΩ of resistance.
Suitable inductors can be purchased from the following
suppliers:
2
2
T
× V
IN
ON
C2 ≥
(inFarads)
(3)
2 ×L × ∆V
× (V
– V )
BOOST
OUT IN
Coilcraft
Coiltronics
Dale
(708) 639-6400
(407) 241-7876
(605) 665-9301
(708) 956-0666
For example, a 2.4V input, a 5V output, a 22µH inductor,
and an allowance of 100mV of ripple on the boost
capacitor results in a minimum C2 value of 15µF.
Sumida
V 4.5V
OUT =
V
5.5V
OUT =
100
95
90
85
80
75
70
65
60
55
50
100
95
90
85
80
75
70
65
60
55
50
L = 47µH
L = 47µH
L = 22µH
L = 22µH
L = 10µH
L = 10µH
1.0
2.0
3.0
(V)
4.0
5.0
1.0
1.5
2.0
2.5
(V)
3.0
3.5
4.0
V
V
IN
IN
V
2.5V
OUT =
V
3.5V
OUT =
100
100
95
90
85
80
75
70
65
60
55
50
95
90
85
80
75
70
65
60
55
50
L = 47µH
L = 47µH
L = 22µH
L = 10µH
L = 22µH
L = 10µH
1.0
1.5
2.0
(V)
2.5
3.0
1.0
1.2
1.4
1.6
(V)
1.8
2.0
V
V
IN
IN
Figure 7. Typical Efficiency at maximum output current as a Function of V .
IN
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
