LTM4604AV View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4604AV' PDF : 22 Pages View PDF
LTM4604A
Applications Information
Run Enable and Soft-Start
The RUN/SS pin provides dual functions of enable and
soft-start control. The RUN/SS pin is used to control turn
on of the LTM4604A. While this pin is below 0.5V, the
LTM4604A will be in a 7µA low quiescent current state. A
0.8V threshold will enable the LTM4604A. This pin can be
used to sequence LTM4604A devices. The voltage on the
RUN/SS pin clamps the control loop’s current comparator
threshold. A RUN/SS pin voltage of 2.375V upon completion
of soft-start guarantees the regulator can deliver full output
current. The soft-start control is provided by a 1M pull-up
resistor (RSS) and a 1000pF capacitor (CSS) as shown in
the Block Diagram. An external capacitor can be applied
to the RUN/SS pin to increase the soft-start time. A typical
value is 0.01µF. Soft-start time is approximately given by:
( ) tSOFTSTART
=
lnï£ï£¬
VIN
VIN
– 1.8V


•
RSS
CSS + CSSEXT
where RSS and CSS are shown in the Block Diagram of
Figure 1, 1.8V is the soft-start upper range, and CSSEXT is
the additional capacitance for further soft-start control. The
soft-start function can also be used to control the output
ramp-up time, so that another regulator can be easily
tracked. An independent ramp control signal can be applied
to the master ramp, otherwise, connect the TRACK pin to
VIN to disable tracking. To turn off the module while VIN
remains active, the RUN/SS pin should be pulled low with
a falling edge ≤1µs to ensure the device does not transition
slowly through the internal undervoltage lockout threshold.
VTRACK is the track ramp applied to the slave’s TRACK pin.
VTRACK applies the track reference for the slave output up
to the point of the programmed value at which VTRACK
proceeds beyond the 0.8V reference value. The VTRACK
VIN
5V
CIN1
10µF
6.3V
X5R OR X7R
CSSEXT
VIN
PGOOD VOUT
LTM4604A
COMP
FB
RUN/SS TRACK
GND
RAMP
CONTROL
OR VIN
RFB3
1.62k
COUT1
22µF
6.3V ×3
X5R OR
X7R
VMASTER
3.3V
4A
VIN
5V
CIN2
10µF
6.3V
X5R OR X7R
VIN
PGOOD VOUT
LTM4604A
COMP
FB
RUN/SS TRACK
GND
RFB2
5.76k
RFB
5.76k
COUT2
22µF
6.3V ×3
X5R OR
X7R
VSLAVE
1.5V
4A
RFB1
4.99k
4604A F02
Figure 2. Dual Outputs (3.3V and 1.5V) with Tracking
Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK pin. The output can be tracked up and
down with another regulator. The master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider
to implement coincident tracking. The LTM4604A uses a
very accurate 4.99k resistor for the top feedback resistor.
Figures 2 and 3 show an example of coincident tracking.
VTRACK
=
RFB2
4.99k + RFB2
•
VMASTER
MASTER OUTPUT
SLAVE OUTPUT
TIME
4604A F03
Figure 3. Output Voltage Coincident Tracking
For more information www.linear.com/LTM4604A
4604afc
11
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