LTC3862 LINER [Linear Technology], LTC3862 Datasheet - Page 13

• Manufacturer Part Number: LTC3862
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

In multi-phase applications, all of the FB pins are connected
together and all of the error amplifi er output pins (ITH) are
connected together. The INTV
be connected together. The INTV
sourcing current but is not capable of sinking current. As
a result, when two or more INTV
connected together, the highest voltage regulator supplies
all of the gate drive and control circuit current, and the
other regulators are off. This would place a thermal burden
on the highest output voltage LDO and could cause the
maximum die temperature to be exceeded. In multi-phase
LTC3862 applications, each INTV
be independently bypassed to its respective PGND pin as
close as possible to each IC.
The Low Voltage Analog and Digital Supply LDO (3V8)
The second LDO within the LTC3862 is powered off of
INTV
and digital control circuitry, as shown in Figure 1. The
output voltage of this LDO (which also has a PMOS out-
put device) is 3.8V. Most of the analog and digital control
circuitry is powered from the internal 3V8 LDO. The 3V8
pin should be bypassed to SGND with a 1nF ceramic ca-
pacitor (X5R or better), placed as close as possible to the
IC pins. This LDO is not intended to be used as a supply
for external circuitry.
Thermal Considerations and Package Options
The LTC3862 is offered in two package options. The 5mm
× 5mm QFN package (UH24) has a thermal resistance
R
thermal resistance of 38°C/W, and the 24-pin SSOP (GN24)
package has a thermal resistance of 85°C/W. The QFN and
TSSOP package options have a lead pitch of 0.65mm, and
the GN24 option has a lead pitch of 0.025in.
The INTV
current. As a result, care must be taken to ensure that
OPERATION
TH(JA)
CC
of 34°C/W, the 24-pin TSSOP (FE24) package has a
and serves as the supply to the low voltage analog
CC
regulator can supply up to 50mA of total
CC
pins, however, should not
CC
CC
CC
regulator output should
regulator is capable of
regulator outputs are
the maximum junction temperature of the IC is never
exceeded. The junction temperature can be estimated
using the following equations:
The total quiescent current (I
supply current (I
the gate capacitance of the power MOSFETs. The value
of Q
Typical Performance Characteristics section of the MOSFET
data sheet. The value listed in the electrical specifi cations
may be measured at a higher V
value of interest is at the 5V INTV
As an example of the required thermal analysis, consider
a 2-phase boost converter with a 9V to 24V input voltage
range and an output voltage of 48V at 2A. The switching
frequency is 150kHz and the maximum ambient tempera-
ture is 70°C. The power MOSFET used for this application
is the Vishay Si7478DP , which has a typical R
8.8mΩ at V
plot of V
50nC (the temperature coeffi cient of the gate charge is
low). One power MOSFET is used for each phase. For the
QFN package option:
In this example, the junction temperature rise is only 14.7°C.
These equations demonstrate how the gate charge current
typically dominates the quiescent current of the IC, and
how the choice of package option and board heat sinking
can have a signifi cant effect on the thermal performance
of the solution.
I
P
T
I
P
T
Q(TOT)
Q(TOT)
J
J
DISS
DISS
G(TOT)
= T
= 70°C + 432mW • 34°C/W = 84.7°C
A
= V
= 24V • 18mA = 432mW
GS
= I
= 3mA + 2 • 50nC • 150kHz = 18mA
+ P
should come from the plot of V
GS
IN
vs Q
Q
DISS
+ Q
• (I
= 4.5V and 7.5mΩ at V
G
Q
Q
G(TOT)
, the total gate charge at V
• R
) and the current required to charge
+ Q
TH(JA)
G(TOT)
• f
Q(TOT)
GS
• f)
, such as 10V, whereas the
CC
) consists of the static
gate drive voltage.
GS
LTC3862
= 10V. From the
GS
vs Q
GS
DS(ON)
13
G
= 5V is
in the
3862fb
of