LTC3734 LINER [Linear Technology], LTC3734 Datasheet - Page 12

• Manufacturer Part Number: LTC3734
• Description: Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3734
The basic LTC3734 application circuit is shown in
Figure 1 on the first page of this data sheet. External
component selection begins with the selection of the
inductors based on ripple current requirements and con-
tinues with the current sensing resistors using the calcu-
lated peak inductor current and/or maximum current
limit. Next, the power MOSFET, D1 is selected. The
operating frequency and the inductor are chosen based
mainly on the amount of ripple current. Finally, C
selected for its ability to handle the input ripple current
and C
output ripple voltage and load step specifications. The
circuit shown in Figure 1 can be configured for operation
up to an input voltage of 28V (limited by the external
MOSFETs).
R
R
current. The LTC3734 current comparator has a maxi-
mum threshold of 72mV/R
mode range of SGND to SV
threshold sets the peak inductor current, yielding a maxi-
mum average output current I
less half the peak-to-peak ripple current, ΔI
Allowing a margin for variations in the LTC3734 and
external component values yields:
Operating Frequency
The LTC3734 uses a constant frequency architecture
with the frequency determined by an internal capacitor.
This capacitor is charged by a fixed current plus an
additional current which is proportional to the DC voltage
applied to the FREQSET pin. The FREQSET voltage is
internally set to 1.2V. It is recommended that this pin is
actively biased with a resistor divider to prevent noise
getting into the system.
A graph for the voltage applied to the FREQSET pin vs
frequency is given in Figure 2. As the operating frequency
is increased the gate drive and switching losses will be
higher, reducing efficiency (see Efficiency Considerations).
The maximum switching frequency is approximately
550kHz.
12
SENSE
SENSE
R
SENSE
OUT
Selection For Output Current
is chosen based on the required peak output
is chosen with low enough ESR to meet the
= (40mV/I
U
MAX
)
U
SENSE
CC
MAX
. The current comparator
and an input common
equal to the peak value
W
L
.
U
IN
is
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because
MOSFET gate charge and transition losses increase
directly with frequency. In addition to this basic tradeoff,
the effect of inductor value on ripple current and low
current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔI
inductance or frequency and increases with higher V
where f is the individual output stage operating frequency.
Accepting larger values of ΔI
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
ΔI
Remember, the maximum ΔI
input voltage. The inductor ripple current is determined by
the frequency, inductance, input and output voltages.
L
ΔI
= 0.4(I
L
=
Figure 2. Operating Frequency vs V
V
OUT
OUT
fL
600
550
500
450
400
350
300
250
200
150
100
), where I
0
⎛
⎜
⎝
1
−
0.5
V
V
OUT
FREQSET PIN VOLTAGE (V)
IN
OUT
1.0
⎞
⎟
⎠
is the maximum load current.
1.5
L
L
L
occurs at the maximum
, decreases with higher
allows the use of low
2.0
2.5
FREQSET
3734 F02
3.0
3734f
IN
: