LTC1700 Linear Technology, LTC1700 Datasheet - Page 13

• Manufacturer Part Number: LTC1700
• Description: No RSENSE Synchronous Step-Up DC/DC Controller
• Manufacturer: Linear Technology

APPLICATIONS
where:
Hence you would select the start-up capacitor, C
ensure t
equation is only valid for V
2.3V, then t
Design Example
Assume the LTC1700 is used to convert a 3.3V input to 5V
output. Load current requirement is a maximum 3A and a
minimum of 100mA. Efficiency at both low and high load
currents is important. Ambient temperature = 25 C.
Since low load current efficiency is important, Burst Mode
operation is enabled by connecting pin 5 to V
Since the duty cycle is less than 36%, the value of the
inductor is chosen based on the L
In the application, (Figure 7) a 4.6 H inductor is used to
further reduce ripple current. The actual ripple current is
now:
For the main N-channel MOSFET, the R
R
DS ON N CHANNEL
V
I
C
Duty Cycle = 1 – V
L
OUT
MINBURST
D
OUT
(
I
L
= Voltage drop of P-channel parasitic diode
)(
= Initial load current during start-up
= Output capacitance
DELAY
3 3
.
POWERUP
V
= 0.8 H.
530
> t
POWERUP.
kHz
U
)
0 34
= 0ns.
IN
.
/V
( .
4 6
I
O MAX
INFORMATION
1
OUT
(
U
IN
–
D
H
< 2.3V. If V
Remember that the above
= 0.34
63
)
)
mV
MINBURST
0 5
0 46
W
. (
.
DS(ON)
A
IN
I
L
)
is greater than
equation.
13 2
OUT
should be:
U
.
.
m
SS
, to
Accounting for the peak current reduction due to slope
compensation (see Figure 5), the R
should be:
The factor, 0.9, is obtained from Figure 5 using a duty cycle
of 34%. The peak current of the inductor is 5A. Select an
inductor that does not saturate at this current level. The
average current through the N-channel MOSFET is 1.62A
while the average current through the synchronous P-
channel MOSFET is 3A.
The FDS6670A and FDS6375 are chosen for the
N-channel and P-channel MOSFET respectively. We can
now calculate the temperature rise in the FDS6670A. RMS
current flowing through the FDS6670A is 2.78A. Hence
power dissipated is:
The
ture rise is:
This is an insignificant temperature rise and therefore the
omission of the T in calculating the required R
does not generate a large error.
At 3A load, the RMS current into the output capacitor is
given by:
To meet the RMS current requirement, two SANYO POSCAP
100 F capacitors are paralleled. These capacitors have
low ESR (55m ) and to futher reduce the overall ESR, a
10 F ceramic capacitor is placed in parallel with the
POSCAP capacitor. Figure 7 shows the complete circuit.
R
T
I
P
COUT(RMS)
RISE
DS(ON)
DISS
JA
of the FDS6670A is 50 C/W. Therefore tempera-
= 61.82mW 50
= 3.1 C
= (2.78)
= 61.82mW
= (13.2)(0.9)
= 11.9m
= 3(5/3.3 – 1)
2
(8 10
–3
0.5
)
= 2.15A
DS(ON)
LTC1700
of the N-channel
13
DS(ON)