LTC3425 Linear Technology, LTC3425 Datasheet - Page 11

• Manufacturer Part Number: LTC3425
• Description: 5A 8MHz 4-Phase Synchronous Step-Up DC/DC Converter
• Manufacturer: Linear Technology

OPERATIO
DETAILED DESCRIPTION
The LTC3425 provides high efficiency, low noise power
for high current boost applications such as cellular phones
and PDAs. The true output disconnect feature eliminates
inrush current and allows V
shutdown. The current mode architecture with adaptive
slope compensation provides ease of loop compensation
with excellent transient load response. The low R
low gate charge synchronous switches eliminate the need
for an external Schottky rectifier, and provide efficient high
frequency pulse width modulation (PWM) control. High
efficiency is achieved at light loads when Burst Mode
operation is entered, where the IC’s quiescent current is a
low 12 A typical on V
MULTIPHASE OPERATION
The LTC3425 uses a 4-phase architecture, rather than the
conventional single phase of other boost converters. By
having multiple phases equally spaced (90 apart), not
only is the output ripple frequency increased by a factor of
four, but the output capacitor ripple current is greatly
reduced. Although this architecture requires four induc-
tors, rather than a single inductor, there are a number of
important advantages.
• Much lower peak inductor current allows the use of
• Greatly reduced output ripple current minimizes output
• Higher frequency output ripple is easier to filter for low
• Input ripple current is also reduced for lower noise on
The peak boost inductor current is given by:
Where I
cycle, N is the number of phases and di is the inductor
ripple current. This relationship is shown graphically in
Figure 1 using a single phase and a 4-phase example.
smaller, lower cost inductors.
capacitance requirement.
noise applications.
V
I
IN
LPEAK
.
O
is the average load current, D is the PWM duty
( – ) •
1
I
D N
O
U
OUT
di
2
.
OUT
to go to zero during
DS(ON)
,
Example:
The following example, operating at 50% duty cycle,
illustrates the advantages of multiphase operation over a
conventional single-phase design.
V
I
Table 1
PARAMETER
Peak-Peak Output
Ripple Current
RMS Output Ripple Current
Peak Inductor Current
Output Ripple Frequency
With 4-phase operation, at least one of the phases will be
delivering current to the load whenever V
one quarter V
duty cycles, there can be as many as two or three phases
delivering load current simultaneously. This greatly re-
duces both the output ripple current and the peak current
in each inductor, compared with a single-phase converter.
This is illustrated in the waveforms of Figures 2 and 3.
Operation Using Only Two or Three Phases
The LTC3425 can operate as a 2- or 3-phase converter by
simply eliminating the inductor from the unused phase(s).
OUT
IN
Figure 1. Comparison of Output Ripple Current with Single
Phase and 4-Phase Boost Converter in a 2A Load Application
Operating at 50% Duty Cycle
= 1.9V, V
= 2A, Frequency = 1MHz, L = 2.2 H
4
3
2
5
1
0
OUT
0
OUT
SINGLE
PHASE
= 3.6V, Efficiency = 90% (approx),
(duty cycles less than 75%). For lower
0.5
FOUR PHASE
SINGLE
4.227A
PHASE
4.227A
2.00A
1MHz
TIME ( s)
www.DataSheet4U.com
1
PHASE
0.450A
0.184A
1.227A
FOUR
4MHz
LTC3425
3425 F01
IN
Reduced by 89%
Reduced by 91%
Reduced by 71%
1.5
Increased by 4
CHANGE FROM
is greater than
1 TO 4 PHASE
11
3425f