Datasheet

LTC3810
28
3810fc
voltage V
OUT2(MIN)
below which continuous operation is
forced until V
OUT2
has risen above its minimum.
V
OUT2(MIN)
= 0.8V 1+
R4
R3






lies the same percentage below the typical value as the
maximum lies above it. Consult the MOSFET manufacturer
for further guidelines.
To further limit current in the event of a short-circuit to
ground, the LTC3810 includes foldback current limiting.
If the output falls by more than 60%, then the maximum
sense voltage is progressively lowered to about one tenth
of its full value.
Be aware also that when the fault timeout is enabled for
the external NMOS regulator, an over current limit may
cause the output to fall below the minimum 6.5V UV
threshold. This condition will cause a linear regulator
timeout/restart sequence as described in the Linear Regula-
tor Timeout section if this condition persists.
Soft-Start and Tracking
The LTC3810 has the ability to either soft-start by itself with
a capacitor or track the output of another supply. When
the device is confi gured to soft-start by itself, a capacitor
should be connected to the TRACK/SS pin. The LTC3810 is
put in a low quiescent current shutdown state (I
Q
~240μA)
if the SHDN pin voltage is below 1.5V. The TRACK/SS
pin is actively pulled to ground in this shutdown state.
Once the SHDN pin voltage is above 1.5V, the LTC3810 is
powered up. A soft-start current of 1.4μA then starts to
charge the soft-start capacitor C
SS
. Note that soft-start
is achieved not by limiting the maximum output current
of the controller but by controlling the ramp rate of the
output voltage. Current foldback is disabled during this
soft-start phase. During the soft-start phase, the LTC3810
is ramping the reference voltage until it reaches 0.8V. The
force continuous mode is also disabled and PGOOD signal
is forced low during this phase. The total soft-start time
can be calculated as:
t
SOFTSTART
= 0.8 • C
SS
/1.4μA
When the device is confi gured to track another supply,
the feedback voltage of the other supply is duplicated
by a resistor divider and applied to the TRACK/SS pin.
Therefore, the voltage ramp rate on this pin is determined
by the ramp rate of the other supply output voltage.
Table 1.
MODE/SYNC PIN CONDITION
DC Voltage: 0V to 0.75V Forced Continuous
Current Reversal Enabled
DC Voltage: ≥ 0.85V Pulse Skip Mode Operation
No Current Reversal
Feedback Resistors Regulating a Secondary Winding
Ext. Clock OV to ≥ 2V Forced Continuous
Current Reversal Enabled
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3810, the maximum sense voltage is controlled
by the voltage on the V
RNG
pin. With valley current control,
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
I
LIMIT
=
V
SNS(MAX)
R
DS(ON)

T
+
1
2
I
L
The current limit value should be checked to ensure that
I
LIMIT(MIN)
> I
OUT(MAX)
. The minimum value of current limit
generally occurs with the largest V
IN
at the highest ambi-
ent temperature, conditions that cause the largest power
loss in the converter. Note that it is important to check for
self-consistency between the assumed MOSFET junction
temperature and the resulting value of I
LIMIT
which heats
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
DS(ON)
of the MOSFETs. The maximum
current limit is determined by the minimum MOSFET
on-resistance. Data sheets typically specify nominal
and maximum values for R
DS(ON)
, but not a minimum.
A reasonable assumption is that the minimum R
DS(ON)
APPLICATIONS INFORMATION