LTM4605
10
4605fd
For more information www.linear.com/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 PLLFLTR is left open, the PLLFLTR pin goes low,
forcing 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 lower. The maximum switching frequency is
approximately 400kHz.
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-locked 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-locked loop function is disabled.
Low Current Operation
To improve the efficiency at low output current operation,
LTM4605 provides three modes for both buck and boost
operations by accepting a logic input on the FCB pin.
Figure 2. Frequency vs PLLFLTR Pin Voltage
PLLFLTR PIN VOLTAGE (V)
0 0.5
OPERATING FREQUENCY (kHz)
2.0
450
400
350
300
250
200
150
100
50
0
4605 F02
1.0 1.5 2.5
Table 2 shows the different operation modes.
Table 2. Different Operating Modes
FCB PIN BUCK BOOST
0V to 0.75V Force Continuous Mode Force Continuous Mode
0.85V to
V
INTVCC
– 1V
Skip-Cycle Mode Burst Mode Operation
>5.3V DCM with Constant Freq 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
V
INTVCC
– 1V, but greater than 0.85V, where V
INTVCC
is
6V, 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 out
-
put 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 INTV
CC
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 C
IN
is driven by the need of filtering the
input square wave current.
