LTC3814-5_15 Datasheet, PDF (13/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
When the controller is operating in continuous mode the
duty cycles for the top and bottom MOSFETs are given by:
Main Switch Duty Cycle = ^{VOUT − VIN}
V_OUT
Synchronous Switch Duty Cycle = ^VIN
V_OUT
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
P_MAIN
=
D_MAX


I_O(MAX)
1
D_MAX

2
(_T
)R_DS(ON)
+
1
2
V_OUT2


I_O(MAX)
1
D_MAX

(R_DR )(C_MILLER )
•

^INTVCC
1
–
V_TH(IL)
+
1
V_TH(IL)



(f)
P_SYNC
=


1

1
D_MAX

(I_O(MAX))²(_T
)
R_DS(0N)
where ρ_T is the temperature dependency of R_DS(ON), R_DR
is the effective top driver resistance (approximately 2Ω at
V_GS = V_MILLER). V_TH(IL) is the data sheet specified typical
gate threshold voltage specified in the power MOSFET
data sheet at the specified drain current. C_MILLER is the
calculated capacitance using the gate charge curve from
the MOSFET data sheet and the technique described above.
Both MOSFETs have I²R losses while the bottom N-channel
equation includes an additional term for transition losses.
Both top and bottom MOSFET I²R losses are greatest at
lowest V_IN, and the top MOSFET I²R losses also peak
during an overcurrent condition when it is on close to
100% of the period. For most LTC3814-5 applications,
the transition loss and I²R loss terms in the bottom
MOSFET are comparable, so best efficiency is obtained
by choosing a MOSFET that optimizes both R_DS(ON) and
C_MILLER. Since there is no transition loss term in the syn-
chronous MOSFET, however, optimal efficiency is obtained
by minimizing R_DS(ON)—by using larger MOSFETs or
paralleling multiple MOSFETs.
Multiple MOSFETs can be used in parallel to lower
R_DS(ON) and meet the current and thermal requirements
if desired. The LTC3814-5 contains large low impedance
drivers capable of driving large gate capacitances without
significantly slowing transition times. In fact, when driv-
ing MOSFETs with very low gate charge, it is sometimes
helpful to slow down the drivers by adding small gate
resistors (10Ω or less) to reduce noise and EMI caused
by the fast transitions.
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 LTC3814-5 applications is
determined implicitly by the one-shot timer that controls
the on-time t_OFF of the synchronous MOSFET switch.
The on-time is set by the current into the I_OFF pin and the
voltage at the V_OFF pin according to:
( ) t_OFF
=
V_VOFF
I_IOFF
76pF
Tying a resistor R_OFF from V_OUT to the I_OFF pin yields a syn-
chronous MOSFET on-time inversely proportional to V_OUT.
This results in the following operating frequency and also
keeps frequency constant as V_OUT ramps up at start-up:
f=
V_IN
(Hz)
V_VOFF • R_OFF (76pF)
The V_OFF pin can be connected to INTV_CC or ground or
can be connected to a resistive divider from V_IN. The V_OFF
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. Note, however, that
if the V_OFF pin is connected to a constant voltage, the
operating frequency will be proportional to the input
voltage V_IN. Figures 4a and 4b illustrate how R_OFF relates
to switching frequency as a function of the input voltage
and V_OFF voltage. To hold frequency constant for input
38145fc
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