Datasheet

LTC3703
28
3703fc
applicaTions inForMaTion
Next, choose the top and bottom MOSFET switch. Since
the drain of each MOSFET will see the full supply voltage
72V (max) plus any ringing, choose a 100V MOSFET to
provide a margin of safety. Si7456DP has a 100V BV
DSS
,
R
DS(ON)
= 25mΩ (max), δ = 0.009/°C, C
MILLER
= (19nC
– 10nC)/50V = 180pF, V
GS(MILLER)
= 4.7V, θ
JA
= 20°C/W.
The power dissipation can be estimated at maximum input
voltage, assuming a junction temperature of 100°C (30°C
above an ambient of 70°C):
P
MAIN
=
12
72
(10)
2
1+ 0.009(100– 25)
[ ]
(0.025)
+(72)
2
10
2






(2)(180pF)•
1
10 – 4.7
+
1
4.7






(250k)
= 0.70W+ 0.94W = 1.64W
And double check the assumed T
J
in the MOSFET:
T
J
= 70°C + (1.64W)(20°C/W) = 103°C
Since the synchronous MOSFET will be conducting over
twice as long each period (almost 100% of the period
in short circuit) as the top MOSFET, use two Si7456DP
MOSFETs on the bottom:
P
SYNC
=
72− 12
72






(10)
2
1+ 0.009(100– 25)
[ ]
•
0.025
2






= 1.74W
T
J
= 70°C + (1.74W)(20°C/W) = 105°C
Next, set the current limit resistor. Since I
MAX
= 10A, the
limit should be set such that the minimum current limit is
>10A. Minimum current limit occurs at maximum R
DS(ON)
.
Using the above calculation for bottom MOSFET T
J
, the
max R
DS(ON)
= (25mΩ/2) [1 + 0.009 (105-25)] = 21.5mΩ.
Therefore, I
MAX
pin voltage should be set to (10A)(0.0215)
= 0.215V. The R
SET
resistor can now be chosen to be
0.215V/12µA = 18k.
C
IN
is chosen for an RMS current rating of about 5A
(I
MAX
/2) at 85°C. For the output capacitor, two low ESR
OS-CON capacitors (18mΩ each) are used to minimize
output voltage changes due to inductor current ripple and
load steps. The ripple voltage will be:
∆V
OUT(RIPPLE)
= ∆I
L(MAX)
(ESR) = (4A)(0.018Ω/2)
= 36mV
However, a 0A to 10A load step will cause an output volt-
age change of up to:
∆V
OUT(STEP)
= ∆I
LOAD(ESR)
= (10A)(0.009Ω) = 90mV
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3703. These items are also illustrated graphically in the
layout
diagram of Figure 18. For layout of a boost mode
converter, layout is similar with V
IN
and V
OUT
swapped.
Check the following in your layout:
1. Keep the signal and power grounds separate. The signal
ground consists of the LTC3703 GND pin, the ground
return of C
VCC
, and the (–) terminal of V
OUT
. The power
ground consists of the Schottky diode anode, the source
of the bottom side MOSFET, and the (–) terminal of the
input capacitor and DRV
CC
capacitor. Connect the signal
and power grounds together at the (–) terminal of the
output capacitor. Also, try to connect the (–) terminal
of the output capacitor as close as possible to the (–)
terminals of the input and DRV
CC
capacitor and away
from the Schottky loop described in (2).
2. The high di/dt loop formed by the top N-channel MOSFET,
the bottom MOSFET and the C
IN
capacitor should have
short leads and PC trace lengths to minimize high fre-
quency noise and voltage stress from inductive ringing.
3. Connect the drain of the top side MOSFET directly to the
(+) plate of C
IN
, and connect the source of the bottom
side MOSFET directly to the (–) terminal of C
IN
. This
capacitor provides the AC current to the MOSFETs.
4. Place
the ceramic C
DRVCC
decoupling capacitor imme-
diately next to the IC, between DRV
CC
and BGRTN. This
capacitor carries the MOSFET drivers’ current peaks.
Likewise the C
B
capacitor should also be next to the IC
between BOOST and SW.