LTC3808 Datasheet, PDF (17/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
satisfied, the capacitance is adequate for filtering. The
output ripple (∆V_OUT) is approximated by:
∆V_OUT
≈
I_RIPPLE
•
⎛
⎝⎜
ESR
+
8•f
1
• C_OUT
⎞
⎠⎟
where f is the operating frequency, C_OUT is the output
capacitance and I_RIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I_RIPPLE increase with input voltage.
Setting Output Voltage
The LTC3808 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 deter-
mined by:
V_OUT
=
0.6V
•
⎛
⎝⎜
1+
R_B
R_A
⎞
⎠⎟
For most applications, a 59k resistor is suggested for R_A.
In applications where minimizing the quiescent current is
critical, R_A should be made bigger to limit the feedback
divider current. If R_B then results in very high impedance,
it may be beneficial to bypass R_B with a 50pF to 100pF
capacitor C_FF.
V_OUT
LTC3808
V_FB
R_B
C_FF
R_A
3808 F03
Figure 3. Setting Output Voltage
3.3V OR 5V
LTC3808
RUN
LTC3808
RUN
3808 F04
Figure 4. RUN Pin Interfacing
Once the controller is enabled, the start-up of V_OUT is
controlled by the state of the TRACK/SS pin. If the TRACK/
SS pin is connected to V_IN, the start-up of V_OUT is con-
trolled by internal soft-start, which slowly ramps the
positive reference to the error amplifier from 0V to 0.6V,
allowing V_OUT to rise smoothly from 0V to its final value.
The default internal soft-start time is around 1ms. The
soft-start time can be changed by placing a capacitor
between the TRACK/SS pin and GND. In this case, the soft-
start time will be approximately:
t_SS
=
C_SS
•
600mV
1µA
where 1µA is an internal current source which is always
on.
When the voltage on the TRACK/SS pin is less than the
internal 0.6V reference, the LTC3808 regulates the V_FB
voltage to the TRACK/SS pin voltage instead of 0.6V.
Therefore the start-up of V_OUT can ratiometrically track an
external voltage V_X, according to a ratio set by a resistor
divider at TRACK/SS pin (Figure 5a). The ratiometric
relation between V_OUT and V_X is (Figure 5c):
V_{OUT =} R_{TA •} R_A + R_B
V_X R_A R_TA + R_TB
Run and Soft-Start/Tracking Functions
The LTC3808 has a low power shutdown mode which is
controlled by the RUN pin. Pulling the RUN pin below 1.1V
puts the LTC3808 into a low quiescent current shutdown
mode (I_Q = 9µA). Releasing the RUN pin, an internal 0.7µA
(at V_IN = 4.2V) current source will pull the RUN pin up to
V_IN, which enables the controller. The RUN pin can be
driven directly from logic as showed in Figure 4.
V_X
R_TB
R_TA
LTC3808
V_FB
TRACK/SS
V_OUT
R_B
R_A
3808 F5a
Figure 5a. Using the TRACK/SS Pin to Track V_X
3808f
17