LTC3808 Datasheet, PDF (14/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3808 F02
Figure 2. R_DS(ON) vs Temperature
which is sensed between the GND and SW pins. The short-
circuit current sense threshold ∆V_SC is set approximately
90mV when IPRG is floating (60mV when IPRG is tied low;
150mV when IPRG is tied high). The on-resistance of N-
channel MOSFET is determined by:
R_DS(ON)MAX
=
∆V_SC
I_SC(PEAK)
The short-circuit current limit (I_SC(PEAK)) should be larger
than the I_OUT(MAX) with some margin to avoid interfering
with the peak current sensing loop. On the other hand, in
order to prevent the MOSFETs from excessive heating and
the inductor from saturation, I_SC(PEAK) should be smaller
than the minimum value of their current ratings. A reason-
able range is:
I_OUT(MAX) < I_SC(PEAK) < I_RATING(MIN)
Therefore, the on-resistance of N-channel MOSFET should
be chosen within the following range:
∆V_SC
I_RATING(MIN)
< R_DS(ON)
<
∆V_SC
I_OUT(MAX)
where ∆V_SC is 90mV, 60mV or 150mV with IPRG being
floated, tied to GND or V_IN respectively.
The power dissipated in the MOSFET strongly depends on
its respective duty cycles and load current. When the
LTC3808 is operating in continuous mode, the duty cycles
for the MOSFETs are:
14
Top P-Channel Duty Cycle = ^VOUT
V_IN
Bottom N-Channel Duty Cycle = ^{VIN – VOUT}
V_IN
The MOSFET power dissipations at maximum output
current are:
P_TOP
=
V_OUT
V_IN
• I_OUT(MAX)2
• ρ_T
• R_DS(ON)
+
2 • V_IN2
• I_OUT(MAX) • C_RSS • f
P_BOT
=
V_IN
– V_OUT
V_IN
• I_OUT(MAX)2
• ρ_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 100%.
The LTC3808 utilizes a non-overlapping, anti-shoot-
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
applications. Many power MOSFETs, particularly P-chan-
nel 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 LTC3808
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.,
3808f