LTM4605 データシートの表示（PDF） - Linear Technology
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LTM4605 Datasheet PDF : 24 Pages
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LTM4605
APPLICATIONS INFORMATION
The PLLFLTR pin can be grounded to lower the frequency
to 200kHz or tied to 2.4V to yield approximately 400kHz.
When PLLIN is left open, the PLLFLTR pin goes low, forc-
ing the oscillator to its minimum frequency.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 2. As the operating frequency
increases, the gate charge losses will be higher, thus the
efficiency is low. The maximum switching frequency is
approximately 400kHz.
450
400
350
300
250
200
150
100
50
0
0
0.5 1.0 1.5 2.0 2.5
PLLFLTR PIN VOLTAGE (V)
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Figure 2. Frequency vs PLLFLTR Pin Voltage
FREQUENCY SYNCHRONIZATION
The LTM4605 can also be synchronized to an external
source via the PLLIN pin instead of adjusting the voltage on
the PLLFLTR pin directly. The power module has a phase-
locked loop comprised of an internal voltage controlled
oscillator and a phase detector. This allows turning on the
internal top MOSFET for locking to the rising edge of the
external clock. A pulse detection circuit is used to detect
a clock on the PLLIN pin to turn on the phase lock loop.
The input pulse width of the clock has to be at least 400ns,
and 2V in amplitude. The synchronized frequency ranges
from 200kHz to 400kHz, corresponding to a DC voltage
input from 0V to 2.4V at PLLFLTR. During the start up of
the regulator, the phase-lock loop function is disabled.
Low Current Operation
To improve the efficiency at low current operation, LTM4605
provides three modes for both buck and boost operations
by accepting a logic input on the FCB pin. Table 2 shows
the different operation modes.
Table 2. Different Operating Modes
FCB PIN
BUCK
0V to 0.75V Force Continuous Mode
0.85V to 5V
Skip-Cycle Mode
>5.3V
DCM with Constant Freq
BOOST
Force Continuous Mode
Burst Mode Operation
DCM with Constant Freq
When the FCB pin voltage is lower than 0.8V, the controller
behaves as a continuous, PWM current mode synchronous
switching regulator. When the FCB pin voltage is below
VINTVCC – 1V, but greater than 0.8V, the controller enters
Burst Mode operation in boost operation or enters skip-
cycle mode in buck operation. During boost operation,
Burst Mode operation is activated if the load current is
lower than the preset minimum output current level. The
MOSFETs will turn on for several cycles, followed by a
variable “sleep” interval depending upon the load current.
During buck operation, skip-cycle mode sets a minimum
positive inductor current level. In this mode, some cycles
will be skipped when the output load current drops below
1% of the maximum designed load in order to maintain
the output voltage.
When the FCB pin is tied to the INTVCC pin, the controller
enters constant frequency discontinuous current mode
(DCM). For boost operation, if the output voltage is high
enough, the controller can enter the continuous current
buck mode for one cycle to discharge inductor current.
In the following cycle, the controller will resume DCM
boost operation. For buck operation, constant frequency
discontinuous current mode is turned on if the preset
minimum negative inductor current level is reached. At
very light loads, this constant frequency operation is not as
efficient as Burst Mode operation or skip-cycle, but does
provide low noise, constant frequency operation.
Input Capacitors
In boost mode, since the input current is continuous, only
minimum input capacitors are required. However, the input
current is discontinuous in buck mode, so the selection
of input capacitor CIN is driven by the need of filtering the
input square wave current.
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