LTC3810-5 Datasheet, PDF (27/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
V_IN
TG
LTC3810-5
R4
SW
FCB
R3
SGND
BG
PGND
+ ^VIN
C_IN
1N4148
•
+
T1 ^• ₊
1:N
V_OUT2
C_OUT2
1μF
V_OUT1
C_OUT
38105 F14
Figure 14. Secondary Output Loop
In addition to providing a logic input to force continu-
ous operation, the MODE/SYNC pin provides a mean to
maintain a flyback winding output when the primary is
operating in pulse skip mode. The secondary output V_OUT2
is normally set as shown in Figure 14 by the turns ratio
N of the transformer. However, if the controller goes into
pulse skip mode and halts switching due to a light primary
load current, then V_OUT2 will droop. An external resistor
divider from V_OUT2 to the MODE/SYNC pin sets a minimum
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


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 LTC3810-5, 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) ρ_T
+
1
2
ΔI_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)
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 LTC3810-5 includes foldback current limiting.
If the output falls by more than 50%, 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 4.5V 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 LTC3810-5 has the ability to either soft-start by itself
with a capacitor or track the output of another supply.
When the device is configured to soft-start by itself, a
capacitor should be connected to the TRACK/SS pin. The
LTC3810-5 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 LTC3810-5 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 maxi-
mum 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 LTC3810-5 is ramping the reference voltage
until it reaches 0.8V. The force continuous mode is also
38105fc
27