LTC3736_15 Datasheet, PDF (15/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE, Synchronous Controller with Output Tracking

LTC3736
APPLICATIO S I FOR ATIO
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3736 F04
Figure 4. R_DS(ON) vs Temperature
The MOSFET power dissipations at maximum output
current are:
P_TOP
=
V_OUT
V_IN
• I_OUT(MAX)2
•r_T
• R_DS(ON)
+2
•
V_IN2
• I_OUT(MAX) • C_RSS • f_OSC
P_BOT
=
V_IN
– V_OUT
V_IN
• I_OUT(MAX)2
•r_T
• R_DS(ON)
Both MOSFETs have I²R losses and the P_TOP equation
includes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest at high input voltage or during a short circuit
when the bottom duty cycle is nearly 100%.
The LTC3736 utilizes a nonoverlapping, antishoot-through
gate drive control scheme to ensure that the P- and
N-channel MOSFETs are not turned on at the same time.
To function properly, the control scheme requires that the
MOSFETs used are intended for DC/DC switching applica-
tions. Many power MOSFETs, particularly P-channel
MOSFETs, are intended to be used as static switches and
therefore are slow to turn on or off.
Reasonable starting criteria for selecting the P-channel
MOSFET are that it must typically have a gate charge (Q_G)
less than 25nC to 30nC (at 4.5V_GS) and a turn-off delay
(t_D(OFF)) of less than approximately 140ns. However, due
to differences in test and specification methods of various
MOSFET manufacturers, and in the variations in Q_G and
t_D(OFF) with gate drive (V_IN) voltage, the P-channel MOSFET
ultimately should be evaluated in the actual LTC3736
application circuit to ensure proper operation.
Shoot-through between the P-channel and N-channel
MOSFETs can most easily be spotted by monitoring the
input supply current. As the input supply voltage in-
creases, if the input supply current increases dramatically,
then the likely cause is shoot-through. Note that some
MOSFETs that do not work well at high input voltages (e.g.,
V_IN > 5V) may work fine at lower voltages (e.g., 3.3V).
Table 1 shows a selection of P-channel MOSFETs from
different manufacturers that are known to work well in
LTC3736 applications.
Selecting the N-channel MOSFET is typically easier, since
for a given R_DS(ON), the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
Table 1. Selected P-Channel MOSFETs Suitable for LTC3736
Applications
PART
NUMBER
MANUFACTURER
TYPE
PACKAGE
Si7540DP
Siliconix
Complementary PowerPak
P/N
SO-8
Si9801DY
Siliconix
Complementary
SO-8
P/N
FDW2520C
Fairchild
Complementary
P/N
TSSOP-8
FDW2521C
Fairchild
Complementary
P/N
TSSOP-8
Si3447BDV
Siliconix
Single P
TSOP-6
Si9803DY
Siliconix
Single P
SO-8
FDC602P
Fairchild
Single P
TSOP-6
FDC606P
Fairchild
Single P
TSOP-6
FDC638P
Fairchild
Single P
TSOP-6
FDW2502P
Fairchild
Dual P
TSSOP-8
FDS6875
Fairchild
Dual P
SO-8
HAT1054R
Hitachi
Dual P
SO-8
NTMD6P02R2-D
On Semi
Dual P
SO-8
Operating Frequency and Synchronization
The choice of operating frequency, f_OSC, is a trade-off
between efficiency and component size. Low frequency
3736fa
15