Datasheet

LTC1871-1
21
18711fb
APPLICATIONS INFORMATION
The inductor ripple current is:
I
L
=  •
I
O(MAX)
1–D
MAX
= 0.4 •
7
1– 0.39
= 4.6A
And so the inductor value is:
L =
V
IN(MIN)
I
L
• f
•D
MAX
=
3.3V
4.6A • 300kHz
• 0.39 = 0.93μH
The component chosen is a 1µH inductor made by
Sumida (part number CEP125-H 1ROMH) which has
a saturation current of greater than 20A.
5. With the input voltage to the IC bootstrapped to the
output of the power supply (5V), a logic-level MOSFET
can be used. Because the duty cycle is 39%, the maxi-
mum SENSE pin threshold voltage is reduced from its
low duty cycle typical value of 150mV to approximately
140mV. Assuming a MOSFET junction temperature of
125°C, the room temperature MOSFET R
DS(ON)
should
be less than:
R
DS(ON)
 V
SENSE(MAX)
•
1–D
MAX
1+

2






•I
O(MAX)
• 
T
= 0.140V •
1– 0.39
1+
0.4
2






•7A•1.5
= 6.8m
The MOSFET used was the Fairchild FDS7760A, which
has a maximum R
DS(ON)
of 8m at 4.5V V
GS
, a BV
DSS
of greater than 30V, and a gate charge of 37nC at 5V
V
GS
.
6. The diode for this design must handle a maximum
DC output current of 10A and be rated for a minimum
reverse voltage of V
OUT
, or 5V. A 25A, 15V diode from
On Semiconductor (MBRB2515L) was chosen for its
high power dissipation capability.
7. The output capacitor usually consists of a high valued
bulk C connected in parallel with a lower valued, low
ESR ceramic. Based on a maximum output ripple voltage
of 1%, or 50mV, the bulk C needs to be greater than:
C
OUT

I
OUT(MAX)
0.01• V
OUT
• f
=
7A
0.01• 5V • 300kHz
= 466μF
The RMS ripple current rating for this capacitor needs
to exceed:
I
RMS(COUT)
I
O(MAX)
•
V
O
–V
IN(MIN)
V
IN(MIN)
=
7A •
5V – 3.3V
3.3V
= 5A
To satisfy this high RMS current demand, four 150µF
Panasonic capacitors (EEFUEOJ151R) are required.
In parallel with these bulk capacitors, two 22µF, low
ESR (X5R) Taiyo Yuden ceramic capacitors (JMK-
325BJ226MM) are added for HF noise reduction.
Check the output ripple with a single oscilloscope
probe connected directly across the output capacitor
terminals, where the HF switching currents fl ow.
8. The choice of an input capacitor for a boost converter
depends on the impedance of the source supply and the
amount of input ripple the converter will safely toler-
ate. For this particular design and lab setup a 100µF
Sanyo Poscap (6TPC 100M), in parallel with two 22µF
Taiyo Yuden ceramic capacitors (JMK325BJ226MM)
is required (the input and return lead lengths are kept
to a few inches, but the peak input current is close to
20A!). As with the output node, check the input ripple
with a single oscilloscope probe connected across the
input capacitor terminals.