LTC3807_15 Datasheet, PDF (19/32 Pages) Linear Technology – Low IQ, Synchronous Step-Down Controller with 24V Output Voltage Capability

LTC3807
APPLICATIONS INFORMATION
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
( )( ) C_IN
Required I_RMS
≈ ^IMAX
V_IN

V_OUT
V_IN – V_OUT ^1/2 (1)
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3807, ceramic capacitors
can also be used for C_IN. Always consult the manufacturer
if there is any question.
A small (0.1μF to 1μF) bypass capacitor between the chip
V_IN pin and ground, placed close to the LTC3807, is also
suggested. A small (≤10Ω) resistor placed between C_IN
(C1) and the V_IN pin provides further isolation.
The selection of C_OUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (ΔV_OUT) is approximated by:
∆V_OUT
≈
∆I_L


ESR
+
8
•
f
1
• C_OUT


where f is the operating frequency, C_OUT is the output
capacitance and ΔI_L is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since ΔI_L increases with input voltage.
Setting Output Voltage
The LTC3807 output voltage is set by an external feedback
resistor divider carefully placed across the output, as shown
in Figure 3. The regulated output voltage is determined by:
V_OUT
=
0.8V


1+
R_B
R_A


To improve the frequency response, a feedforward ca-
pacitor, C_FF, may be used. Great care should be taken to
route the V_FB line away from noise sources, such as the
inductor or the SW line.
V_OUT
LTC3807
V_FB
R_B
C_FF
R_A
3807 F03
Figure 3. Setting Output Voltage
RUN Pin
The LTC3807 is enabled using the RUN pin. It has a rising
threshold of 1.21V with 50mV of hysteresis. Pulling the
RUN pin below 1.16V shuts down the main control loop.
Pulling it below 0.7V disables the controller and most
internal circuits, including the INTV_CC LDOs. In this state,
the LTC3807 draws only 14μA of quiescent current.
Releasing the RUN pin allows a 7μA internal current to
pull up the pin to enable the controller. The RUN pin may
be externally pulled up or driven directly by logic. When
driving the RUN pin with a low impedance source, do not
exceed the absolute maximum rating of 8V. The RUN pin
has an internal 11V voltage clamp that allows the RUN pin
to be connected through a resistor to a higher voltage (for
example, V_IN), so long as the maximum current into the
RUN pin does not exceed 100μA.
The RUN pin can be implemented as a UVLO by connecting
it to the output of an external resistor divider network off
V_IN, as shown in Figure 4.
V_IN
LTC3807
R_B
RUN
R_A
3807 F04
Figure 4. Using the RUN Pin as a UVLO
For more information www.linear.com/LTC3807
3807f
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