LTC3611_15 Datasheet, PDF (17/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

LTC3611
Applications Information
INTV_CC
3.3V OR 5V
D1
V_IN
RUN/SS
R_SS*
R_SS*
D2* ^RUN/SS
C_SS
2N7002
C_SS
(5a)
3611 F05
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
(5b)
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
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. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3611 circuits:
1. DC I²R losses. These arise from the resistance of the
internal resistance of the MOSFETs, inductor and PC board
traces and cause the efficiency to drop at high output
currents. In continuous mode the average output current
flows through L, but is chopped between the top and bot-
tom MOSFETs. If the two MOSFETs have approximately
the same R_DS(ON), then the DC I²R loss for one MOSFET
can simply be determined by [R_DS(ON) + R_L] • I_O.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V and can be estimated from:
Transition Loss ≅ (1.7A^–1) V_IN2 I_OUT C_RSS f
3. INTV_CC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by sup-
plying INTV_CC current through the EXTV_CC pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.
4. C_INloss.Theinputcapacitorhasthedifficultjoboffiltering
the large RMS input current to the regulator. It must have
a very low ESR to minimize the AC I²R loss and sufficient
capacitance to prevent the RMS current from causing ad-
ditional upstream losses in fuses or batteries.
Other losses, including C_OUT ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If
you make a change and the input current decreases, then
the efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
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 load current. When
a load step occurs, V_OUT immediately shifts by an amount
equal to ΔI_LOAD (ESR), where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT generating a feedback error signal used by the
regulator to return V_OUT to its steady-state value. During
this recovery time, V_OUT can be monitored for overshoot
or ringing that would indicate a stability problem. The I_TH
pin external components shown in Figure 6 will provide
adequate compensation for most applications. For a
detailed explanation of switching control loop theory see
Application Note 76.
3611fd
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