LTC3826-1 Datasheet, PDF (16/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3826-1
APPLICATIONS INFORMATION
This is why the input capacitor’s requirement calculated
above for the worst-case controller is adequate for the dual
controller design. Also, the input protection fuse resistance,
battery resistance, and PC board trace resistance losses
are also reduced due to the reduced peak currents in a
2-phase system. The overall benefit of a multiphase design
will only be fully realized when the source impedance of the
power supply/battery is included in the efficiency testing.
The sources of the top MOSFETs should be placed within
1cm of each other and share a common C_IN(s). Separating
the sources and C_IN may produce undesirable voltage and
current resonances at V_IN.
A small (0.1μF to 1μF) bypass capacitor between the chip V_IN
pin and ground, placed close to the LTC3826-1, is also sug-
gested. A 10Ω resistor placed between C_IN (C1) and the V_IN
pin provides further isolation between the two channels.
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_RIPPLE


ESR
+
1
8fC_OUT


where f is the operating frequency, C_OUT is the output
capacitance and I_RIPPLE is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since I_RIPPLE increases with input voltage.
Setting Output Voltage
The LTC3826-1 output voltages are each 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 feed-forward
capacitor, 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.
SENSE⁺ and SENSE^– Pins
The common mode input range of the current comparator
is from 0V to 10V. Continuous linear operation is provided
throughout this range allowing output voltages from 0.8V
to 10V. The input stage of the current comparator requires
that current either be sourced or sunk from the SENSE pins
depending on the output voltage, as shown in the curve in
Figure 4. If the output voltage is below 1.5V, current will
flow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V_OUT
resistor divider to compensate for the current comparator’s
negative input bias current. Since V_FB is servoed to the
0.8V reference voltage, R_A in Figure 3 should be chosen
to be less than 0.8V/I_SENSE, with I_SENSE determined from
Figure 4 at the specified output voltage.
V_OUT
1/2 LTC3826-1
V_FB
R_B
C_FF
R_A
38261 F03
Figure 3. Setting Output Voltage
60
30
0
–30
–60
–90
–120
–150
–180
–210
–240
–270
–300
0
1 2 3 4 5 6 7 8 9 10
V_SENSE COMMON MODE VOLTAGE (V)
38261 F04
Figure 4. SENSE Pins Input Bias Current
vs Common Mode Voltage
38261fb
16