LTM4608 LINER [Linear Technology], LTM4608 Datasheet - Page 10

• Manufacturer Part Number: LTM4608
• Description: Low VIN, 8A DC/DC ?ModuleTM with Tracking, Margining, and Frequency Synchronization
• Manufacturer: LINER [Linear Technology]

LTM4608
OPERATION
The typical LTM4608 application circuit is shown in
Figure 18. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 3 for specifi c external capacitor
requirements for a particular application.
V
There are restrictions in the maximum V
down ratio that can be achieved for a given input voltage.
The LTM4608 is 100% duty cycle, but the V
minimum drop out is still shown as a function of its load
current. For 5V input, all outputs can deliver 8A. For 3.3V
input, all outputs can deliver 8A, except 2.5V which is
limited to 6A.
Output Voltage Programming
The PWM controller has an internal 0.596V reference
voltage. As shown in the Block Diagram, a 10kΩ/0.5%
internal feedback resistor connects V
together. The output voltage will default to 0.596V with
no feedback resistor. Adding a resistor R
to GND programs the output voltage:
Current mode control provides cycle-by-cycle fast current
limit and thermal shutdown in an overcurrent condition.
Internal overvoltage and undervoltage comparators pull the
open-drain PGOOD output low if the output feedback volt-
age exits a ±10% window around the regulation point.
Pulling the RUN pin below 1.3V forces the controller into
its shutdown state, by turning off both M1 and M2 at low
load current. The TRACK pin is used for programming the
output voltage ramp and voltage tracking during start-up.
See Applications Information.
The LTM4608 is internally compensated to be stable over
all operating conditions. Table 3 provides a guideline for
input and output capacitances for several operating con-
ditions. The Linear Technology μModule Power Design
Tool is provided for transient and stability analysis. The
FB pin is used to program the output voltage with a single
external resistor to ground.
APPLICATIONS INFORMATION
10
IN
to V
OUT
Step-Down Ratios
OUT
IN
FB
to V
and FB pins
from FB pin
IN
OUT
to V
step-
OUT
Multiphase operation can be easily employed with the
synchronization and phase mode controls. Up to 12 phases
can be cascaded to run simultaneously with respect to each
other by programming the PHMODE pin to different levels.
The LTM4608 has clock in and clock out for poly phasing
multiple devices or frequency synchronization.
High effi ciency at light loads can be accomplished with
selectable Burst Mode operation using the MODE pin. These
light load features will accommodate battery operation.
Effi ciency graphs are provided for light load operation in
the Typial Performance Characteristics.
Output voltage margining is supported, and can be pro-
gramed from ±5% to ±15% using the MGN and BSEL pins.
The PGOOD pin is disabled during margining
Table 2. R
Input Capacitors
The LTM4608 module should be connected to a low AC
impedance DC source. Three 10μF ceramic capacitors
are included inside the module. Additional input capaci-
tors are only needed if a large load step is required up to
the 4A level. A 47μF to 100μF surface mount aluminum
electrolytic bulk capacitor can be used for more input bulk
capacitance. This bulk input capacitor is only needed if
the input source impedance is compromised by long in-
ductive leads, traces or not enough source capacitance.
If low impedance power planes are used, then this 47μF
capacitor is not needed.
For a buck converter, the switching duty-cycle can be
estimated as:
V
R
V
OUT
FB
OUT
=
FB
0.596V
Open
0 596
Resistor vs Output Voltage
.
V
1.2V
10k
•
10
k R
R
+
FB
6.65k
1.5V
FB
4.87k
1.8V
3.09k
2.5V
2.21k
3.3V
4608f