LTC3642 Datasheet, PDF (13/20 Pages) Linear Technology – High Effi ciency, High Voltage 50mA Synchronous Step-Down Converter

LTC3642
APPLICATIONS INFORMATION
The resistive divider allows the V_FB pin to sense a fraction
of the output voltage as shown in Figure 6.
V_FB
LTC3642
GND
V_OUT
R1
R2
Figure 6. Setting the Output Voltage
To minimize the no-load supply current, resistor values in
the megohm range should be used. The increase in supply
current due to the feedback resistors can be calculated
from:
ΔI_VIN
=
⎛
⎝⎜
V_OUT ⎞
R1+ R2⎠⎟
•
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
Run Pin with Programmable Hysteresis
The LTC3642 has a low power shutdown mode controlled
by the RUN pin. Pulling the RUN pin below 0.7V puts the
LTC3642 into a low quiescent current shutdown mode
(I_Q ~ 3μA). When the RUN pin is greater than 1.2V, the
controller is enabled. Figure 7 shows examples of con-
figurations for driving the RUN pin from logic.
V_SUPPLY
V_IN
LTC3642
RUN
4.7M LTC3642
RUN
3642 F07
Figure 7. RUN Pin Interface to Logic
The RUN pin can alternatively be configured as a precise
undervoltage lockout (UVLO) on the V_IN supply with a
resistive divider from V_IN to ground. The RUN pin com-
parator nominally provides 10% hysteresis when used in
this method; however, additional hysteresis may be added
with the use of the HYST pin. The HYST pin is an open-
drain output that is pulled to ground whenever the RUN
comparator is not tripped. A simple resistive divider can
be used as shown in Figure 8 to meet specific V_IN voltage
requirements.
V_IN
R1
R2
R3
RUN
LTC3642
HYST
3642 F08
Figure 8. Adjustable Undervoltage Lockout
Specific values for these UVLO thresholds can be computed
from the following equations:
Risin g
V_IN
UVLO
Threshold =
1.21V
•
⎛
⎝⎜
1+
R1⎞
R2 ^⎟⎠
Fallin g
V_IN
UVLO
Threshold =
1.10V
•
⎛
⎝⎜
1+
R1 ⎞
R2 + R3⎠⎟
The minimum value of these thresholds is limited to the
internal V_IN UVLO thresholds that are shown in the Electri-
cal Characteristics table. The current that flows through
this divider will directly add to the shutdown, sleep and
active current of the LTC3642, and care should be taken to
minimize the impact of this current on the overall efficiency
of the application circuit. Resistor values in the megohm
range may be required to keep the impact on quiescent
shutdown and sleep currents low. Be aware that the HYST
pin cannot be allowed to exceed its absolute maximum
rating of 6V. To keep the voltage on the HYST pin from
exceeding 6V, the following relation should be satisfied:
V_IN(MAX)
•
⎛
⎝⎜
R1+
R3
R2 +
⎞
R3 ⎠⎟
<
6V
The RUN pin may also be directly tied to the V_IN supply
for applications that do not require the programmable
undervoltage lockout feature. In this configuration, switch-
ing is enabled when V_IN surpasses the internal undervoltage
lockout threshold.
Soft-Start
The internal 0.75ms soft-start is implemented by ramping
both the effective reference voltage from 0V to 0.8V and the
peak current limit set by the I_SET pin (25mA to 115mA).
3642f
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