Datasheet

LT1683
21
1683fd
APPLICATIONS INFORMATION
current controls the fundamental component. ESR should
also be low to reduce capacitor dissipation.
The capacitance value can be computed by consideration
of desired load ripple, duty cycle and ESR.
C
OUT
=
1
∆ V
OUT
∆I
L(MAX)
−ESR
•
DC
MIN
f
MOSFET Selection
There is a wide variety of MOSFETs to choose from for
this part. The part will work with either normal threshold
(3V to 4V) or logic-level threshold devices (1V to 2V).
Select a voltage rating to ensure under worst-case condi-
tions that the MOSFET will not break down. Next choose
an R
ON
sufficiently low to meet both the power dissipation
capabilities of the MOSFET package as well as overall ef-
ficiency needs of the converter.
The LT1683 can handle a large range of gate charges.
However at very large charge stability may be affected.
The power dissipation in the MOSFET depends on several
factors. The primary element is I
2
R heating when the device
is on. In addition, power is dissipated when the device is
slewing. An estimate for power dissipation is:
PV
I
I
I
VR I
I
IN
SR
IN
ON
=
+
+
− +
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
•
Δ
•
•Δ
2
2
2
2
2
2
4
3
4
⎦⎦
⎥
⎥
⎧
⎨
⎪
⎪
⎩
⎪
⎪
⎫
⎬
⎪
⎪
⎭
⎪
⎪
+
V
I
f I R DC
SR
ON
•
• ••
2
where I is the average current, ∆I is the ripple current in
the switch, I
SR
is the current slew rate, V
SR
is the voltage
slew rate, f is the oscillator frequency, DC is the duty cycle
and R
ON
is the MOSFET on-resistance.
Setting GCL Voltage
Setting the voltage on the GCL pin depends on what type
of MOSFET is used and the desired gate drive undervoltage
lockout voltage.
First determine the maximum gate drive that you require.
Typically you will want it to be at least 2V greater than the
maximum threshold. Higher voltages will lower the on
resistance and increase efficiency. Be certain to check the
maximum allowed gate voltage. Often this is 20V but for
some logic threshold MOSFETs it is only 8V to 10V.
V
GCL
needs to be set approximately 0.2V above the desired
max gate threshold. In addition V
IN
needs to be at least
1.6V above the gate voltage.
The GCL pin can be tied to V
IN
which will result in a maxi-
mum gate voltage of V
IN
– 1.6V.
This pin also controls undervoltage lockout of the gate
drives. The undervoltage lockout will prevent the MOSFETs
from switching until there is sufficient drive present.
If GCL is tied to a voltage source or Zener less than 6.8V,
the gate drivers will not turn on until V
IN
exceeds the GCL
voltage by 0.8V. For V
GCL
above 6.5V, the gate drives are
ensured to be off for V
IN
< 7.3V and they will be turned
on by V
GCL
+ 0.8V.
If GCL is tied to V
IN
, the gate drivers are always on
(undervoltage lockout is disabled).
Approximately 50µA of current can be sourced from this pin
if V
IN
> V
GCL
+ 0.8V. This could be used to bias a Zener.
The GCL pin has an internal 19V Zener to ground that will
provide a failsafe for maximum gate voltage.
As an example say we are using a Siliconix Si4480DY
which has R
DS(ON)
rated at 6V. To get 6V, V
GCL
needs to
be set to 6.2V and V
IN
needs to be at least 7.6V.