LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 15

• Manufacturer Part Number: LTC1929
• Description: 2-Phase, High Efficiency, Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Minimum On-Time Considerations
Minimum on-time t
that the LTC1929 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty cycle
applications may approach this minimum on-time limit
and care should be taken to ensure that
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1929 will begin to skip
cycles resulting in nonconstant frequency operation. The
output voltage will continue to be regulated, but the ripple
current and ripple voltage will increase.
The minimum on-time for the LTC1929 is generally less
than 200ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases. This is of
particular concern in forced continuous applications with
low ripple current at light loads. If the duty cycle drops
below the minimum on-time limit in this situation, a
significant amount of cycle skipping can occur with corre-
spondingly larger current and voltage ripple.
PLLIN
t
ON MIN
EXTERNAL
OSC
Figure 7. Phase-Locked Loop Block Diagram
50k
V f
V
IN
OUT
FREQUENCY
DETECTOR
DETECTOR
U
ON(MIN)
DIGITAL
PHASE/
PHASE
U
is the smallest time duration
2.4V
W
PLLFLTR
R
10k
LP
U
1929 F07
OSC
C
LP
If an application can operate close to the minimum on-
time limit, an inductor must be chosen that has a low
enough inductance to provide sufficient ripple amplitude
to meet the minimum on-time requirement. As a general
rule, keep the inductor ripple current of each phase equal
to or greater than 15% of I
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:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1929 circuits: 1) LTC1929 V
cluding loading on the differential amplifier output),
2) INTV
MOSFET transition losses.
1) The V
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the differential
amplifier output. V
(<0.1%) loss.
2) INTV
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from INTV
ground. The resulting dQ/dt is a current out of INTV
is typically much larger than the control circuit current. In
continuous mode, I
are the gate charges of the topside and bottom side
MOSFETs.
%Efficiency = 100% – (L1 + L2 + L3 + ...)
CC
CC
IN
regulator current, 3) I
current is the sum of the MOSFET driver and
current has two components: the first is the
IN
GATECHG
current typically results in a small
OUT(MAX)
= (Q
2
T
R losses and 4) Topside
+ Q
at V
B
), where Q
LTC1929
IN(MAX)
IN
current (in-
.
T
15
and Q
CC
CC
that
to
B