LTC3708_15 Datasheet, PDF (14/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
APPLICATIONS INFORMATION
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3708 F03
Figure 3. R_DS(ON) vs Temperature
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and the
load current. When the LTC3708 is operating in continuous
mode, the duty cycles for the MOSFETs are:
D_TOP
=
V_OUT
V_IN
D_BOT
=
V_IN
– V_OUT
V_IN
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
P_TOP = D_TOP • I_OUT(MAX)2 • ρ_T(TOP) • R_DS(ON) +
(0.5) • V_IN2 • I_OUT(MAX) • C_RSS • f •
( ) ^⎛
R_DR • _⎜
⎝
DRV_CC
1
– V_GS(TH)
+
1
V_GS(TH)
⎞
⎟
⎠
P_BOT = D_BOT • I_OUT(MAX)2 • ρ_T(BOT) • R_DS(ON)
Both MOSFETs have I²R losses and the top MOSFET in-
cludes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest when the bottom duty cycle is near 100%,
during a short circuit or at high input voltage.
Operating Frequency
The choice of operating frequency is a trade-off between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching and
driving losses but requires larger inductance and/or ca-
pacitance to maintain low output ripple voltage.
The operating frequency of LTC3708 applications is deter-
mined implicitly by the one-shot timer that controls the on
time, t_ON, of the top MOSFET switch. The on time is set
by the current into the I_ON pin according to:
( ) ^t_ON
=
0.7
I_ION
10pF
Tying a resistor, R_ON, from V_IN to the I_ON pin yields an
on time inversely proportional to V_IN. For a step-down
converter, this results in approximately constant frequency
operation as the input supply varies:
f
=
0.7
V_OUT
• R_ON(10pF ₎
Figure 4 shows how R_ON relates to switching frequency
for several common output voltages.
1000
V_OUT = 1.5V
V_OUT = 3.3V
V_OUT = 2.5V
100
100
1000
R_ON (kΩ)
10000
3708 F04
Figure 4. Switching Frequency vs R_ON
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