LTC3704_15 Datasheet, PDF (11/28 Pages) Linear Technology – Wide Input Range, No RSENSE Positive-to-Negative DC/DC Controller

LTC3704
APPLICATIO S I FOR ATIO
cause the LTC3704 to exceed its maximum junction
temperature rating. The junction temperature can be
estimated using the following equations:
I_Q(TOT) ≈ I_Q + f • Q_G
P_IC = V_IN • (I_Q + f • Q_G)
T_J = T_A + P_IC • R_TH(JA)
The total quiescent current I_Q(TOT) consists of the static
supply current (I_Q) and the current required to charge and
discharge the gate of the power MOSFET. The 10-pin
MSOP package has a thermal resistance of R_TH(JA) =
120°C/W.
As an example, consider a power supply with V_IN = 5V and
V_SW(MAX) = 12V. The switching frequency is 500kHz, and
the maximum ambient temperature is 70°C. The power
MOSFET chosen is the IRF7805, which has a maximum
R_DS(ON) of 11mΩ (at room temperature) and a maximum
total gate charge of 37nC (the temperature coefficient of
the gate charge is low).
I_Q(TOT) = 600μA + 37nC • 500kHz = 19.1mA
P_IC = 5V • 19.1mA = 95mW
T_J = 70°C + 120°C/W • 95mW = 81.4°C
This demonstrates how significant the gate charge current
can be when compared to the static quiescent current in
the IC.
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in a continuous mode at high V_IN. A tradeoff
between the operating frequency and the size of the power
MOSFET may need to be made in order to maintain a
reliable IC junction temperature. Prior to lowering the
operating frequency, however, be sure to check with
power MOSFET manufacturers for their latest-and-great-
est low Q_G, low R_DS(ON) devices. Power MOSFET manu-
facturing technologies are continually improving, with
newer and better performance devices being introduced
almost yearly.
Output Voltage Programming
The output voltage is set by a resistor divider according to
the following formula:
V_O = V_REF • ^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟} + I_NFB •R2
where V_REF = –1.230V, and I_NFB is the current which flows
out of the NFB pin (I_NFB = –7.5μA). In order to properly
dimension R2, including the effect of the NFB pin current,
the following formula can be used:
R2
=
V_OUT
⎛⎝⎜
V_REF
R1
− V_REF
+ I_NFB^{⎞⎠⎟}
The nominal 7.5μA current which flows out of the NFB pin
has a production tolerance of approximately ±2.5μA, so an
output divider current of 500μA (R1 = 2.49k) results in a
0.5% uncertainty in the output voltage. For low power
applications where the output voltage tolerance is less
important, efficiency can be increased by increasing the
value of R1.
Programming Turn-On and Turn-Off Thresholds
with the RUN Pin
The LTC3704 contains an independent, micropower volt-
age reference and comparator detection circuit that re-
mains active even when the device is shut down, as shown
in Figure 8. This allows users to accurately program an
input voltage at which the converter will turn on and off.
The falling threshold voltage on the RUN pin is equal to the
internal reference voltage of 1.248V. The comparator has
100mV of hysteresis to increase noise immunity.
The turn-on and turn-off input voltage thresholds are
programmed using a resistor divider according to the
following formulas:
V_IN(OFF) = 1.248V • ^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
V_IN(ON) = 1.348V • ^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
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