LTC3720_15 Datasheet, PDF (11/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

LTC3720
APPLICATIO S I FOR ATIO
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3720 F02
Figure 2. R_DS(ON) vs. Temperature
the load current. When the LTC3720 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)(MAX)
+ k V_IN2 I_OUT(MAX) C_RSS f
P_BOT = D_BOT I_OUT(MAX)2 ρ_T(BOT) R_DS(ON)(MAX)
Both MOSFETs have I²R losses and the top MOSFET
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A^–1 can
be used to estimate the amount of transition loss. 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 tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3720 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 and the voltage at the V_ON
pin according to:
t_ON
=
V_VON
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 con-
verter, this results in approximately constant frequency
operation as the input supply varies:
[ ] ^{f =}
V_OUT
Hz
V_VON R_ON(10pF)
To hold frequency constant during output voltage changes,
tie the V_ON pin to V_OUT. The V_ON pin has internal clamps
that limit its input to the one-shot timer. If the pin is tied
below 0.7V, the input to the one-shot is clamped at 0.7V.
Similarly, if the pin is tied above 2.4V, the input is clamped
at 2.4V.
Because the voltage at the I_ON pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
V_IN, especially in applications with lower input voltages.
To correct for this error, an additional resistor R_ON2
connected from the I_ON pin to the 5V INTV_CC supply will
further stabilize the frequency.
R_ON2
=
5V
0.7V
R_ON
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I_TH pin to the V_ON pin and V_OUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I_TH pin to the V_ON pin as
3720f
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