LTC4008
16
4008fb
APPLICATIONS INFORMATION
Selection criteria for the power MOSFETs include the “ON”
resistance R
DS(ON)
, total gate capacitance Q
G
, reverse
transfer capacitance C
RSS
, input voltage and maximum
output current. The charger is operating in continuous
mode so the duty cycles for the top and bottom MOSFETs
are given by:
Main Switch Duty Cycle = V
OUT
/V
IN
Synchronous Switch Duty Cycle = (V
IN
– V
OUT
)/V
IN
.
The MOSFET power dissipations at maximum output
current are given by:
PMAIN = V
OUT
/V
IN(IMAX)
2
(1 + δΔT)R
DS(ON)
+ k(V
IN
)
2
(I
MAX
)(C
RSS
)(f
OSC
)
PSYNC = (V
IN
– V
OUT
)/V
IN
(I
MAX
)
2
(1 + δΔT)R
DS(ON)
Where δΔT is the temperature dependency of R
DS(ON)
and
k is a constant inversely related to the gate drive current.
Both MOSFETs have I
2
R losses while the PMAIN equation
includes an additional term for transition losses, which
are highest at high input voltages. For V
IN
< 20V the high
current effi ciency 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
RSS
actually provides higher effi ciency. The synchronous
MOSFET losses are greatest at high input voltage or during
a short circuit when the duty cycle in this switch in nearly
100%. The term (1 + δΔT) is generally given for a MOSFET
in the form of a normalized R
DS(ON)
vs temperature curve,
but δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs. C
RSS
= Q
GD
/ΔV
DS
is usually specifi ed
in the MOSFET characteristics. The constant k = 2 can be
used to estimate the contributions of the two terms in the
main switch dissipation equation.
If the charger is to operate in low dropout mode or with
a high duty cycle greater than 85%, then the topside P-
channel effi ciency generally improves with a larger MOSFET.
Using asymmetrical MOSFETs may achieve cost savings
or effi ciency gains.
The Schottky diode D1, shown in the Typical Application
on the back page, conducts during the dead-time between
the conduction of the two power MOSFETs. This prevents
the body diode of the bottom MOSFET from turning on and
storing charge during the dead-time, which could cost as
much as 1% in effi ciency. A 1A Schottky is generally a good
size for 4A regulators due to the relatively small average
current. Larger diodes can result in additional transition
losses due to their larger junction capacitance.
The diode may be omitted if the effi ciency loss can be
tolerated.
Calculating IC Power Dissipation
The power dissipation of the LTC4008 is dependent upon
the gate charge of the top and bottom MOSFETs (Q
G1
&
Q
G2
respectively) The gate charge is determined from the
manufacturer’s data sheet and is dependent upon both
the gate voltage swing and the drain voltage swing of the
MOSFET. Use 6V for the gate voltage swing and V
DCIN
for
the drain voltage swing.
PD = V
DCIN
• (f
OSC
(Q
G1
+ Q
G2
) + I
Q
)
Example:
V
DCIN
= 19V, f
OSC
= 345kHz, Q
G1
= Q
G2
= 15nC,
I
Q
= 5mA
PD = 292mW
Adapter Limiting
An important feature of the LTC4008 is the ability to auto-
matically adjust charging current to a level which avoids
overloading the wall adapter. This allows the product to
operate at the same time that batteries are being charged
without complex load management algorithms. Addition-
ally, batteries will automatically be charged at the maximum
possible rate of which the adapter is capable.
This feature is created by sensing total adapter output cur-
rent and adjusting charging current downward if a preset
adapter current limit is exceeded. True analog control is
