LTC3735EUHF Linear Technology, LTC3735EUHF Datasheet - Page 12

• Manufacturer Part Number: LTC3735EUHF
• Description: IC CTRLR DC/DC 2PH HI EFF 38-QFN
• Manufacturer: Linear Technology

Specifications of LTC3735EUHF

• Applications: Controller, Intel Mobile CPU
• Number Of Outputs: 1
• Voltage - Output: 0.7 ~ 1.71 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 38-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Input: -

LTC3735
APPLICATIO S I FOR ATIO
The basic LTC3735 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 MOSFETs, D1 and D2 are selected.
The operating frequency and the inductor are chosen
based mainly on the amount of ripple current. Finally, C
is selected for its ability to handle the input ripple current
(that PolyPhase
sen with low enough ESR to meet the output ripple voltage
and load step specifications (also minimized with
PolyPhase). Current mode architecture provides inherent
current sharing between 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 maxi-
mum threshold of 72mV/R
mode range of SGND to PV
threshold sets the peak inductor current, yielding a maxi-
mum average output current I
less half the peak-to-peak ripple current, ΔI
Assuming a common input power source for each output
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
12
SENSE
SENSE1,2
R
SENSE
Selection For Output Current
are chosen based on the required peak output
= 2(40mV/I
TM
operation minimizes) and C
U
MAX
)
U
SENSE
CC
MAX
. The current comparator
and an input common
equal to the peak value
SENSE
W
resistors.
L
.
OUT
U
is cho-
IN
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.
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 PolyPhase
approach reduces both input and output ripple currents
while optimizing individual output stages to run at a lower
fundamental frequency, enhancing efficiency.
PolyPhase is a registered trademark of Linear Technology Corporation.
Figure 2. Operating Frequency vs V
600
550
500
450
400
350
300
250
200
150
100
0
0.5
FREQSET PIN VOLTAGE (V)
1.0
1.5
2.0
2.5
3735 F02
3.0
FREQSET
3735f