SC4505EVB SEMTECH [Semtech Corporation], SC4505EVB Datasheet - Page 8

• Manufacturer Part Number: SC4505EVB
• Description: High Efficiency Boost Converter for Backlight/Flash LED Driver
• Manufacturer: SEMTECH [Semtech Corporation]

When using tiny inductors for size-sensitive applications,
only a few limited selections are available that fulfill this
need, and all are within the low H range of inductance.
We select a Coiltronics 1.5 H SD3114 series, presently
the only 3mmx3mmx1.5mm inductor that can provide
the required current in the microminiature size required
in space-constrained applications.
Table 4 shows a list of several low profile inductor
manufacturers. Please consult the manufacturers for
detailed information on their entire selection of power
inductors.
Output Capacitor Selection
The next task in SC4505 design is targeting the proper
amount of ripple voltage due to the constant-current LED
loads. The two error amplifiers that control the PWM
converter sense the delta between requested current
and actual current in each output current regulator. On
a cycle-by-cycle basis, a small amount of output ripple
ensures good sensing and tight regulation, while the
output current regulators keep each LED current at a
fixed value. Overall, this allows usage of small output
caps while ensuring precision LED current regulation.
POWER MANAGEMENT
Applications Information
IHLP2525CZ1R5
IHLP2525CZ3R3
IHLP2525CZ4R7
LQH3C4R7M24
LQH3C100M24
CMD4D06-4R7
2006 Semtech Corp.
CLQ4D10-4R7
CLQ4D10-6R8
LB2016B4R7
LB2016B100
SD3112-1R0
SD3114-1R5
SD3114-4R7
PART
I
PK
0.8A
Table 4. Recommended Inductors
(µH)
4.7
4.7
4.7
6.8
4.7
4.7
6.8
1.5
3.3
4.7
1.0
1.5
10
L
1.5
2.7V
µ
H
0.069
0.057
0.147
0.260
0.300
0.250
0.350
0.216
0.162
0.195
0.028
0.037
0.014
MAX
DCR
(Ω )
1
µ
S
HEIGHT
(mm)
MAX
1.45
1.45
1.2
2.2
2.2
1.6
1.6
0.8
1.2
1.2
3.0
3.0
3.0
81%
www.cooperet.com
www.murada.com
www.t-yuden.com
www.sumida.com
www.vishay.com
1
2
Taiyo Yuden
Coiltronics
VENDOR
Sumida
Murata
Vishay
1.5A
8
Although the mechanics of regulation and frequency
dependence may be complex, actual selection of output
capacitor can be simplified to two boundary conditions,
minimum output current and maximum output current.
Output capacitor is chosen to keep ripple voltage
between 10mV & 200mV under all loads.
Design example for Backlight=20mA, Flashlight=100mA:
Minimum Load Current: 20mA (Backlight Only)
Maximum Load Current: 120mA (Backlight and Flash)
All other cases (Torch mode only, Flash mode only,
Backlight and Torch) fall within these two boundary
conditions, so they are automatically satisfied by the
selected output capacitor.
Since the load is a constant current, the capacitor
equation
ripple.
At 1MHz switching frequency and with the assumption
of the worse case analysis (D=0), an even simpler
relationship can be applied:
where C
For worse case analysis, We see that our typical case of
20mA, 120mA can be immediately converted into its
corresponding ripple relationships of:
where C
where C
For the example, if 1 F output capacitor were used, the
20mV/120mV boundary conditions are well within the
suggested guidelines.
Recommended ceramic capacitor manufacturers are
listed in Table 5.
OUT
OUT
OUT
is in F.
is in F.
is in F.
I
C
V
V
OUT
V
OUTMAX
OUTMIN
V
V
T
OUT
C
can be solved for the output
I
= 120mV/C
OUT
= 20mV/C
OU
C
I
T
OUT
OU
1
T
µ
S
1
µ
OUT
OUT
S
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SC4505