LTC3642 Linear Technology, LTC3642 Datasheet - Page 13

• Manufacturer Part Number: LTC3642
• Description: High Voltage 50mA Synchronous Step-Down Converter
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
The resistive divider allows the V
of the output voltage as shown in Figure 6.
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:
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
controller is enabled. Figure 7 shows examples of con-
fi gurations for driving the RUN pin from logic.
The RUN pin can alternatively be confi gured as a precise
undervoltage lockout (UVLO) on the V
resistive divider from V
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 specifi c V
requirements.
Q
ΔI
~ 3μA). When the RUN pin is greater than 1.2V, the
VIN
V
SUPPLY
=
⎛
⎝ ⎜
Figure 7. RUN Pin Interface to Logic
Figure 6. Setting the Output Voltage
R
V
1
OUT
+
RUN
LTC3642
R
2
LTC3642
⎞
⎠ ⎟
•
IN
GND
⎛
⎝ ⎜
V
FB
V
to ground. The RUN pin com-
V
OUT
IN
⎞
⎠ ⎟
V
FB
OUT
R1
R2
pin to sense a fraction
V
IN
4.7M
IN
RUN
supply with a
LTC3642
3642 F07
IN
voltage
Specifi c values for these UVLO thresholds can be computed
from the following equations:
The minimum value of these thresholds is limited to the
internal V
cal Characteristics table. The current that fl ows 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 effi ciency
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 satisfi ed:
The RUN pin may also be directly tied to the V
for applications that do not require the programmable
undervoltage lockout feature. In this confi guration, switch-
ing is enabled when V
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
Ri
Fall g V UVLO Threshold
sin
V
in
IN MAX
g V UVLO Threshold
(
IN
IN
IN
Figure 8. Adjustable Undervoltage Lockout
)
UVLO thresholds that are shown in the Electri-
•
⎛
⎝ ⎜
R
1
+
R
R
IN
V
IN
2
3
surpasses the internal undervoltage
R1
R2
R3
+
R
3
RUN
HYST
⎞
⎠ ⎟
LTC3642
SET
<
=
=
3642 F08
1 21
6
1 10
pin (25mA to 115mA).
V
.
.
V
V
•
LTC3642
•
⎛
⎝ ⎜
⎛
⎝ ⎜
1
1
+
+
R
R
R
2 ⎟ ⎟
1
2
IN
⎞
⎠
R
+
13
supply
1
R
3 3
3642f
⎞
⎠ ⎟