ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 26

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

LTC1876
APPLICATIO S I FOR ATIO
Low V
In applications where the input supply is low (<5V), the
LTC1876 auxiliary regulator can be used to step-up the
input to provide the gate drive to the external MOSFETs as
shown in Figure 9.
Shown in the Typical Application section of the data sheet
is a circuit (3.3V
Supply) with input supply of 3.3V. The boost section of the
LTC1876 is set up to generate 5V and is used to provide the
gate drive to the external MOSFETs. The circuit provides
dual outputs, a 2.5V/15A and 1.8V/15A. Both drawing
power directly from V
Single Output/High Current Applications
In applications that demand current much higher than a
single stage can supply (>20A), the LTC1876 can be
configured as a single output converter. Figure 10 shows
the block diagram of the configuration. Note that the
compensation pins (I
connected together, saving a set of passive components.
In addition, the output voltage sense pins (V
V
divider to set the output voltage.
Although the output current requirement is high, the input
capacitors ripple current requirement is not much differ-
ent compared to the dual outputs circuit. This is attributed
to the fact that the current is shared between two channels
and an out-of-phase architecture is implemented for the
controllers (See Theory and Benefits of 2-Phase
Operation).
26
OSENSE2
SUPPLY
INPUT
IN
LTC1876
SECTION
AUXV
BOOST
SGND
Applications
AUXV
) are shorted together, using only one resistor
AUXSW
IN
FB
Figure 9. Generating the Gate Drive
for Low Input Supply Applications
L1
D1
IN
U
Dual-Phase High Efficiency Power
TH1
IN
R8
R7
+
.
and I
U
C
OUT
TH2
V
STEP-DOWN
IN
LTC1876
SECTION
) of the two channels are
W
EXTERNAL
MOSFETs
OSENSE1
U
1876 F09
and
Auxiliary Regulator’s Inductor Value Calculation
Since the current limit for the auxiliary regulator is inter-
nally set at 1A, it makes the selection of components
easier. For the boost regulator, the duty cycle is given by:
Since energy is only transferred to the output capacitor(s)
during the off-time, the maximum output current that can
be supplied by the regulator without losing regulation is:
where I
1A.
With the required ripple current determined, the value of
the inductor is:
where f = operating frequency (1.2MHz)
In most cases, a larger value of inductance is used. This is
done to account for component variation. It also lowers
the inductor ripple current and results in lower core
losses. In addition, lower ripple also translates into lower
ESR losses in the output capacitors and smaller output
voltage ripple.
I
Duty Cycle
L
OUT
R
I
SUPPLY
C
L
R1
INPUT
C
C
= inductor’s ripple current
(
= 0.5(2 • I
PK
V
IN
R2
= peak inductor current and is internally set at
Figure 10. Single Output Configuration
•
( •
V
V
I
I
TH2
TH1
Duty Cycle
f
OSENSE1
OSENSE2
LTC1876
SGND
V
IN
1–
I
PK
L
)
V
– I
V
OUT
IN
)
L
EXTERNAL
MOSFETs
)(1 – Duty Cycle)
AND SENSE1
AND SENSE2
TO SENSE2
TO SENSE1
L2
L1
+
–
+
–
R
R
S2
S1
1876 F10
+
V
1876fa
OUT