LTC1143L-ADJ_15 Datasheet, PDF (12/20 Pages) Linear Technology – Dual High Efficiency SO-16 Step-Down Switching Regulator Controllers

LTC1143/LTC1143L
LTC1143L-ADJ
APPLICATIONS INFORMATION
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalums, available
in case heights ranging from 2mm to 4mm. For example,
if 200µF/10V is called for in an application requiring 3mm
height, (2) AVX 100µF/10V (P/N TPSD 107M010) could be
used. Consult the manufacturer for other specific
recommendations.
At low supply voltages a minimum capacitance at C_OUT is
needed to prevent an abnormal low frequency operating
mode (see Figure 4). When C_OUT is made too small the
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode operation
to be activated when the LTC1143 series would normally
be in continuous operation. The output remains in
regulation at all times.
1000
800
600
L = 50µH
R_SENSE = 0.02Ω
L = 25µH
R_SENSE = 0.02Ω
400
L = 50µH
200
R_SENSE = 0.05Ω
0
0
1
2
3
4
5
V_IN – V_OUT VOLTAGE (V)
1143 F04
Figure 4. Minimum Value of C_OUT
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, V_OUT shifts by an
amount equal to ∆I_LOAD × ESR, where ESR is the effective
series resistance of C_OUT. ∆I_LOAD also begins to charge or
discharge C_OUT until the regulator loop adapts to the
current change and returns V_OUT to its steady-state value.
During this recovery time V_OUT can be monitored for
overshoot or ringing which would indicate a stability
problem. The Pin 15(7) external components shown in the
Figure 1 circuit will prove adequate compensation for
most applications.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C_OUT causing a rapid drop in V_OUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately 25 • C_LOAD.
Thus a 10µF capacitor would require a 250µs rise time,
limiting the charging current to about 200mA.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power. (For high efficiency circuits only small
errors are incurred by expressing losses as a percentage
of output power.)
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1143 series circuits:
1) DC bias current
2) MOSFET gate charge current
3) I²R losses
4) Voltage drop of the Schottky diode.
1) The DC supply current is the current that flows into
V_IN (Pin 13 and Pin 5) less the gate charge current. For
V_IN = 10V the DC supply current for each section is 160µA
for no load and increases proportionally with load up to
a constant 1.6mA after the LTC1143 series has entered
continuous mode. Because the DC bias current is
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