ltc3717-1 Linear Technology Corporation, ltc3717-1 Datasheet - Page 15

• Manufacturer Part Number: ltc3717-1
• Description: Wide Operating Range, No Rsense Step-down Controller For Ddr/qdr Memory Termination
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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 LTC3717-1 circuits:
1. DC I
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 bottom MOSFETs. If the two
MOSFETs have approximately the same R
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
loss. For example, if R
loss will range from 1% up to 10% as the output current
varies from 1A to 10A for a 1.5V output.
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 capaci-
tance, among other factors. The loss is significant at input
voltages above 20V and can be estimated from:
3. INTV
and control currents.
4. C
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
sufficient capacitance to prevent the RMS current from
causing additional upstream losses in fuses or batteries.
Other losses, including C
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.
Transition Loss (1.7A
IN
loss. The input capacitor has the difficult job of
2
CC
R losses. These arise from the resistances of the
current. This is the sum of the MOSFET driver
U
DS(ON)
OUT
U
–1
= 0.01 and R
) V
ESR loss, Schottky diode D1
IN
2
W
I
OUT
C
DS(ON)
RSS
L
= 0.005 , the
2
f
U
R loss and
, then the
2
R
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
equal to I
resistance of C
discharge C
by the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability
problem. The I
Figure 1 will provide adequate compensation for most
applications. For a detailed explanation of switching
control loop theory see Application Note 76.
Design Example
As a design example, take a supply with the following
specifications: V
1.25V 5%, I
the timing resistor with V
and choose the inductor for about 40% ripple current at
the maximum V
Selecting a standard value of 0.68 H results in a maximum
ripple current of:
Next, choose the synchronous MOSFET switch. Choosing
a Si4874 (R
JA
V
R
L
= 40 C/W) yields a nominal sense voltage of:
SNS(NOM)
ON
I
L
(
250
(
( .
250
0 7
LOAD
kHz
OUT
DS(ON)
OUT(MAX)
= (10A)(1.3)(0.0083 ) = 108mV
1 25 2 5
1 25
V
kHz
.
.
)( . )(
1 25
)(
TH
IN
(ESR), where ESR is the effective series
OUT
generating a feedback error signal used
IN
.
250
0 4 10
:
)( .
V
V
= V
pin external components shown in
. I
0 68
( .
= 0.0083
OUT
V
kHz
REF
= 10A, f = 250kHz. First, calculate
LOAD
V
immediately shifts by an amount
A
)(
ON
H
– .
)
= 2.5V, V
10
)
OUT
0 7
1
= V
pF
OUT
1
also begins to charge or
–
V
to its steady-state value.
OUT
) .
1 25
2 5
(NOM) 0.010
2 5
)
1 25
.
2 5
.
can be monitored for
.
.
:
V
V
EXTVCC
V
V
LTC3717-1
V
514
0 63
= 5V, V
3 7
.
k
.
sn37171 37171fs
A
H
15
(MAX),
OUT
=