LTC3642
APPLICATIONS INFORMATION
V
The resistive divider allows the V pin to sense a fraction
IN
FB
of the output voltage as shown in Figure 6.
R1
RUN
LTC3642
HYST
V
OUT
R2
R3
R1
V
3642 F08
FB
R2
LTC3642
GND
Figure 8. Adjustable Undervoltage Lockout
Figure 6. Setting the Output Voltage
SpecificvaluesfortheseUVLOthresholdscanbecomputed
from the following equations:
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:
⎛
⎞
R1
R2
Rising V UVLO Threshold = 1.21V • 1+
IN
⎜
⎟
⎠
⎝
⎛
⎞
⎛
⎝
⎞
⎠
⎛
⎝
⎞
VOUT
R1+R2
VOUT
R1
R2+R3
Falling V UVLO Threshold = 1.10V • 1+
ΔIVIN
=
•
IN
⎜
⎟
⎠
⎜
⎟
⎜
⎟
V
⎝
⎠
IN
The minimum value of these thresholds is limited to the
Run Pin with Programmable Hysteresis
internal V UVLO thresholds that are shown in the Electri-
IN
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
minimizetheimpactofthiscurrentontheoverallefficiency
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:
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
(IQ ~ 3ꢀA). 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
V
IN
SUPPLY
4.7M
LTC3642
RUN
LTC3642
RUN
⎛
⎞
R3
3642 F07
V
•
< 6V
IN(MAX)
⎜
⎝
⎟
⎠
R1+R2+R3
Figure 7. RUN Pin Interface to Logic
The RUN pin may also be directly tied to the V supply
IN
for applications that do not require the programmable
The RUN pin can alternatively be configured as a precise
undervoltagelockoutfeature.Inthisconfiguration,switch-
undervoltage lockout (UVLO) on the V supply with a
IN
ingisenabledwhenV surpassestheinternalundervoltage
IN
resistive divider from V to ground. The RUN pin com-
IN
lockout threshold.
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
Soft-Start
The internal 0.75ms soft-start is implemented by ramping
boththeeffectivereferencevoltagefrom0Vto0.8Vandthe
peak current limit set by the I pin (25mA to 115mA).
SET
be used as shown in Figure 8 to meet specific V voltage
requirements.
IN
3642f
13
Linear [ Linear ]
