LTC1628IG-PG#PBF Linear Technology, LTC1628IG-PG#PBF Datasheet - Page 13

• Manufacturer Part Number: LTC1628IG-PG#PBF
• Description: IC SW REG STEP-DOWN 28-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1628IG-PG#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Adj to 0.8V
• Current - Output: 3A
• Frequency - Switching: 220kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

OPERATIO
APPLICATIO S I FOR ATIO
the RMS input current varies for single-phase and 2-phase
operation for 3.3V and 5V regulators over a wide input
voltage range.
It can readily be seen that the advantages of 2-phase
operation are not just limited to a narrow operating range,
but in fact extend over a wide region. A good rule of thumb
for most applications is that 2-phase operation will reduce
Figure 1 on the first page is a basic LTC1628 application
circuit. External component selection is driven by the
load requirement, and begins with the selection of R
and the inductor value. Next, the power MOSFETs and D1
are selected. Finally, C
circuit shown in Figure 1 can be configured for operation
up to an input voltage of 28V (limited by the external
MOSFETs).
R
R
The LTC1628 current comparator has a maximum thresh-
old of 75mV/R
SGND to 1.1(INTV
sets the peak of the inductor current, yielding a maximum
average output current I
half the peak-to-peak ripple current, I
Allowing a margin for variations in the LTC1628 and
external component values yields:
SENSE
SENSE
Selection For Output Current
is chosen based on the required output current.
3.0
2.5
2.0
1.5
1.0
0.5
Figure 4. RMS Input Current Comparison
0
0
SENSE
V
V
O1
O2
= 5V/3A
= 3.3V/3A
U
CC
U
10
and an input common mode range of
). The current comparator threshold
(Refer to Functional Diagram)
INPUT VOLTAGE (V)
DUAL CONTROLLER
IN
MAX
U
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
20
and C
equal to the peak value less
OUT
30
W
are selected. The
L
1628 F04
.
40
U
SENSE
the input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
A final question: If 2-phase operation offers such an
advantage over single-phase operation for dual switching
regulators, why hasn’t it been done before? The answer is
that, while simple in concept, it is hard to implement.
Constant-frequency current mode switching regulators
require an oscillator derived “slope compensation” signal
to allow stable operation of each regulator at over 50%
duty cycle. This signal is relatively easy to derive in single-
phase dual switching regulators, but required the develop-
ment of a new and proprietary technique to allow 2-phase
operation. In addition, isolation between the two channels
becomes more critical with 2-phase operation because
switch transitions in one channel could potentially disrupt
the operation of the other channel.
The LTC1628 is proof that these hurdles have been sur-
mounted. The new device offers unique advantages for the
ever-expanding number of high efficiency power supplies
required in portable electronics.
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability crite-
rion for buck regulators operating at greater than 50%
duty factor. A curve is provided to estimate this reducton
in peak output current level depending upon the operating
duty factor.
Selection of Operating Frequency
The LTC1628 uses a constant frequency architecture with
the frequency determined by an internal oscillator capaci-
tor. This internal capacitor is charged by a fixed current
plus an additional current that is proportional to the
voltage applied to the FREQSET pin.
A graph for the voltage applied to the FREQSET pin vs
frequency is given in Figure 5. As the operating frequency
R
SENSE
LTC1628/LTC1628-PG
50
I
MAX
mV
13
1628fb