Datasheet

LTC3876
24
3876f
The MOSFET power dissipations at maximum output
current are given by:
P
TOP
=D
TOP
•I
OUT(MAX)
2
•R
DS(ON)(MAX)
1+δ
()
+ V
IN
2
•
I
OUT(MAX)
2
⎛
⎝
⎜
⎞
⎠
⎟
•C
MILLER
R
TG(HI)
V
DRVCC
–V
MILLER
+
R
TG(LO)
V
MILLER
⎡
⎣
⎢
⎤
⎦
⎥
•f
P
BOT
= D
BOT
• I
OUT(MAX)
2
• R
DS(ON)(MAX)
• (1 + δ )
where δ is the temperature dependency of R
DS(ON)
, R
TG(HI)
is the TG pull-up resistance, and R
TG(LO)
is the TG pull-
down resistance. V
MILLER
is the Miller effect V
GS
voltage
and is taken graphically from the MOSFET ’s data sheet.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
IN
< 20V,
the high current efficiency generally improves with larger
MOSFETs, while for V
IN
> 20V, the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
MILLER
actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve in the
power MOSFET data sheet. For low voltage MOSFETs,
0.5% per degree (°C) can be used to estimate δ as an
approximation of percentage change of R
DS(ON)
:
δ = 0.005/°C • (T
J
– T
A
)
where T
J
is estimated junction temperature of the MOSFET
and T
A
is ambient temperature.
C
IN
Selection
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle V
OUT
/
V
IN
. To prevent large voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The worst-case RMS current occurs by assuming
a single-phase application. The maximum RMS capacitor
current is given by:
I
RMS
≅I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
–1
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that capacitor
manufacturers’ ripple current ratings are often based on
only 2000 hours of life. This makes it advisable to further
derate the capacitor or to choose a capacitor rated at a
higher temperature than required. Several capacitors may
also be paralleled to meet size or height requirements in
the design. Due to the high operating frequency of the
LTC3876, additional ceramic capacitors should also be
used in parallel for C
IN
close to the IC and power switches
to bypass the high frequency switching noises. Typically
multiple X5R or X7R ceramic capacitors are put in parallel
with either conductive-polymer or aluminum-electrolytic
types of bulk capacitors. Because of its low ESR, the ce-
ramic capacitors will take most of the RMS ripple current.
Vendors do not consistently specify the ripple current
rating for ceramics, but ceramics could also fail due to
excessive ripple current. Always consult the manufacturer
if there is any question.
Figure 6 represents a simplified circuit model for calculat-
ing the ripple currents in each of these capacitors. The
input inductance (L
IN
) between the input source and the
input of the converter will affect the ripple current through
APPLICATIONS INFORMATION
+
+
–
L
IN
1µH
ESR
(BULK)
V
IN
ESL
(BULK)
C
IN(BULK)
ESR
(CERAMIC)
ESL
(CERAMIC)
I
PULSE(PHASE1)
C
IN(CERAMIC)
I
PULSE(PHASE2)
3876 F06
Figure 6. Circuit Model for Input Capacitor
Ripple Current Simulation