LTC3735 LINER [Linear Technology], LTC3735 Datasheet - Page 12

• Manufacturer Part Number: LTC3735
• Description: 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs
• Manufacturer: LINER [Linear Technology]

LTC3735
APPLICATIONS INFORMATION
The basic LTC3735 application circuit is shown in
Figure 1 on the first page of this data sheet. External com-
ponent selection begins with the selection of the inductors
based on ripple current requirements and continues with
the current sensing resistors using the calculated peak
inductor current and/or maximum current limit. Next, the
power MOSFETs, D1 and D2 are selected. The operating
frequency and the inductor are chosen based mainly on
the amount of ripple current. Finally, C
ability to handle the input ripple current (that PolyPhase
operation minimizes) and C
ESR to meet the output ripple voltage and load step
specifications (also minimized with PolyPhase). Current
mode architecture provides inherent current sharing be-
tween output stages. The circuit shown in Figure 1 can
be configured for operation up to an input voltage of 28V
(limited by the external MOSFETs). Current mode control
allows the ability to connect the two output stages to two
different input power supply rails. A heavy output load
can take some power from each input supply according
to the selection of the R
R
R
current. The LTC3735 current comparator has a maximum
threshold of 72mV/R
range of SGND to PV
sets the peak inductor current, yielding a maximum aver-
age output current I
the peak-to-peak ripple current, ∆I
Assuming a common input power source for each out-
put stage and allowing a margin for variations in the
LTC3735 and external component values yields:
Operating Frequency
The LTC3735 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
12
SENSE
SENSE1,2
R
SENSE
Selection For Output Current
are chosen based on the required peak output
= 2(40mV/I
MAX
CC
SENSE
MAX
. The current comparator threshold
SENSE
equal to the peak value less half
)
OUT
and an input common mode
resistors.
is chosen with low enough
L
.
IN
is selected for its
®
biased with a resistor divider to prevent noise getting
into the system.
A graph for the voltage applied to the FREQSET pin vs fre-
quency 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.
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 cur-
rent operation must also be considered. The PolyPhase
approach reduces both input and output ripple currents
while optimizing individual output stages to run at a lower
fundamental frequency, enhancing efficiency.
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I
inductance or frequency and increases with higher V
where f is the individual output stage operating frequency.
∆I
L
=
V
Figure 2. Operating Frequency vs V
600
550
500
450
400
350
300
250
200
150
100
OUT
fL
0

 
1−
0.5
FREQSET PIN VOLTAGE (V)
V
V
OUT
IN
1.0

 
1.5
L
, decreases with higher
2.0
2.5
3735 F02
FREQSET
3.0
3735fa
IN
: