LTC3622_15 Datasheet, PDF (12/24 Pages) Linear Technology – 17V, Dual 1A Synchronous Step-Down Regulator with Ultralow Quiescent Current

LTC3622/
LTC3622-2/LTC3622-23/5
Applications Information
Output Voltage Programming
For non-fixed output voltage parts, the output voltage is
set by external resistive dividers according to the follow-
ing equation:
V_OUT = 0.6V • ^{}1+ ^{RR21}
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 2.
For fixed V_OUT parts, tie FB directly to V_OUT, as R2 and R1
are matched internal resistors.
V_OUT
R2
C_FF
LTC3622 ^FB
R1
GND
3622 F02
Figure 2. Setting the Output Voltage
Input Capacitor (C_IN) Selection
The input capacitance, C_IN, is needed to filter the square
wave current at the drain of the top power MOSFET. To
prevent large voltage transients from occurring, a low
ESR input capacitor sized for the maximum RMS current
should be used. The RMS current calculation is different
if the part is used in in-phase or out-of-phase.
For "in phase", when V_OUT1 = V_OUT2
V_OUT(V_IN – V_OUT )
V_IN
This formula has a maximum at V_IN = 2V_OUT. This simple
worst case is commonly used to determine the highest I_RMS.
For out-of-phase case, the ripple current can be lower than
the "in phase" current. The maximum current typically oc-
curs when V_OUT1 – V_IN/2 = V_OUT2 or when V_OUT2 – V_IN/2
= V_OUT1. As a good rule of thumb, the amount of worst
case ripple is about 75% of the worst case ripple in the
in-phase mode. Also note that when V_OUT1 = V_OUT2 = V_IN/2
and I1 = I2, the input current ripple is at its minimum.
Note that ripple current ratings from capacitor manufac-
turers are often based on only 2000 hours of life which
makes it advisable to further derate the capacitor, or choose
a capacitor rated at a higher temperature than required.
Several capacitors may also be paralleled to meet size or
height requirements in the design. For low input voltage
applications, sufficient bulk input capacitance may be
needed to minimize transient effects during output load
changes.
Output Capacitor (C_OUT) Selection
The selection of C_OUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response. The output ripple, ΔV_OUT, is
determined by:
ΔV_OUT
<

ΔI_L


8
•
ƒ
1
• C_OUT

+ ESR_

The output ripple is highest at maximum input voltage since
ΔI_L increases with input voltage. Multiple capacitors placed
in parallel may be needed to meet the ESR and RMS current
handling requirements. Dry tantalum, special polymer and
hybrid conductive polymer capacitors are very low ESR but
have lower capacitance density than other types. Tantalum
capacitors have the highest capacitance density but it is
importance to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications provided that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteristics
and small footprints.
Using Ceramic Input and Output Capacitors
Higher capacitance value, lower cost ceramic capacitors
are now becoming available in smaller case sizes. Their
high ripple current, high voltage rating and low ESR make
them ideal for switching regulator applications. However,
care must be taken when these capacitors are used at
the input and output. When a ceramic capacitor is used
3622fa
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