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LTC3717EUH-1#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
13
LTC3717-1
sn37171 37171fs
APPLICATIO S I FOR ATIO
WUUU
250ns. The minimum off-time limit imposes a maximum
duty cycle of t
ON
/(t
ON
+ t
OFF(MIN)
). If the maximum duty
cycle is reached, due to a dropping input voltage for
example, then the output will drop out of regulation. The
minimum input voltage to avoid dropout is:
VV
tt
t
IN MIN OUT
ON OFF MIN
ON
()
()
=
+
Output Voltage Programming
When V
FB
is connected to V
OUT
, the output voltage is
regulated to one half of the voltage at the V
REF
pin. A
resistor connected between V
FB
and V
OUT
can be used to
further adjust the output voltage according to the follow-
ing equation:
VV
kR
k
OUT REF
FB
=
+
60
120
If V
REF
exceeds 3V, resistors should be placed in series
with the V
REF
pin and the V
FB
pin to avoid exceeding the
input common mode range of the internal error amplifier.
To maintain the V
OUT
= V
REF
/2 relationship, the resistor in
series with the V
REF
pin should be made twice as large as
the resistor in series with the V
FB
pin.
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3717-1 as well as a timer for soft-start and overcur-
rent latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3717-1 into a low quiescent current shutdown
(I
Q
< 30µA). Releasing the pin allows an internal 1.2µA
current source to charge up the external timing capacitor
C
SS
. If RUN/SS has been pulled all the way to ground,
there is a delay before starting of about:
t
V
A
CsFC
DELAY SS SS
=
µ
=µ
()
15
12
13
.
.
./
When the voltage on RUN/SS reaches 1.5V, the LTC3717-
1 begins operating with a clamp on I
TH
of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
TH
is raised until its full 2.4V range is available. This takes an
additional 1.3s/µF, during which the load current is folded
back. During start-up, the maximum load current is re-
duced until either the RUN/SS pin rises to 3V or the output
reaches 75% of its final value. The pin can be driven from
logic as shown in Figure 6. Diode D1 reduces the start
delay while allowing C
SS
to charge up slowly for the soft-
start function.
Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated
Figure 5. Optional External Gate Driver
V
FB
V
REF
LTC3717-1
37171 F04
V
OUT
R
FB
249k
R
FB
499k
V
REF
Figure 4
External Gate Drive Buffers
The LTC3717-1 drivers are adequate for driving up to
about 30nC into MOSFET switches with RMS currents of
50mA. Applications with larger MOSFET switches or oper-
ating at frequencies requiring greater RMS currents will
benefit from using external gate drive buffers such as the
LTC1693. Alternately, the external buffer circuit shown in
Figure 5 can be used. Note that the bipolar devices reduce
the signal swing at the MOSFET gate.
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
(6a) (6b)
D2*
C
SS
R
SS
*
C
SS
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
R
SS
*
37171 F06
14
LTC3717-1
sn37171 37171fs
After the controller has been started and given adequate
time to charge up the output capacitor, C
SS
is used as a
short-circuit timer. After the RUN/SS pin charges above
4V, if the output voltage falls below 75% of its regulated
value, then a short-circuit fault is assumed. A 1.8µA cur-
rent then begins discharging C
SS
. If the fault condition
persists until the RUN/SS pin drops to 3.5V, then the con-
troller turns off both power MOSFETs, shutting down the
converter permanently. The RUN/SS pin must be actively
pulled down to ground in order to restart operation.
The overcurrent protection timer requires that the soft-
start timing capacitor C
SS
be made large enough to guar-
antee that the output is in regulation by the time C
SS
has
reached the 4V threshold. In general, this will depend upon
the size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor can
be estimated from:
C
SS
> C
OUT
V
OUT
R
SENSE
(10
–4
[F/V s])
Generally 0.1µF is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. The feature can be overridden by adding a pull-
up current greater than 5µA to the RUN/SS pin. The
additional current prevents the discharge of C
SS
during a
fault and also shortens the soft-start period. Using a
resistor to V
IN
as shown in Figure 6a is simple, but slightly
increases shutdown current. Connecting a resistor to
INTV
CC
as shown in Figure 6b eliminates the additional
shutdown current, but requires a diode to isolate C
SS
. Any
pull-up network must be able to pull RUN/SS above the
4.2V maximum threshold of the latchoff circuit and over-
come the 4µA maximum discharge current.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3717-1. The INTV
CC
pin can supply up to
50mA RMS and must be bypassed to ground with a
minimum of 4.7µF tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers. Applica-
tions using large MOSFETs with a high input voltage and
APPLICATIO S I FOR ATIO
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high frequency of operation may cause the LTC3717-1 to
exceed its maximum junction temperature rating or RMS
current rating. Most of the supply current drives the
MOSFET gates unless an external EXTV
CC
source is used.
In continuous mode operation, this current is I
GATECHG
=
f(Q
g(TOP)
+ Q
g(BOT)
). The junction temperature can be
estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3717EUH-1
is limited to less than 14mA from a 30V supply:
T
J
= 70°C + (14mA)(30V)(34°C/W) = 84.3°C
For larger currents, consider using an external supply with
the EXTV
CC
pin.
EXTV
CC
Connection
The EXTV
CC
pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTV
CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
CC
pin to INTV
CC
. INTV
CC
power is supplied from EXTV
CC
until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV
CC
pin and ensure that EXTV
CC
≤ V
CC
. The follow-
ing list summarizes the possible connections for EXTV
CC
:
1. EXTV
CC
grounded. INTV
CC
is always powered from the
internal 5V regulator.
2. EXTV
CC
connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV
CC
connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
15
LTC3717-1
sn37171 37171fs
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3717-1 circuits:
1. DC I
2
R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R
DS(ON)
, then the
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
2
R
loss. For example, if R
DS(ON)
= 0.01Ω and R
L
= 0.005Ω, the
loss will range from 1% up to 10% as the output current
varies from 1A to 10A for a 1.5V output.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the input
voltage, load current, driver strength and MOSFET capaci-
tance, among other factors. The loss is significant at input
voltages above 20V and can be estimated from:
Transition Loss ≅ (1.7A
–1
) V
IN
2
I
OUT
C
RSS
f
3. INTV
CC
current. This is the sum of the MOSFET driver
and control currents.
4. C
IN
loss. The input capacitor has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R loss and
sufficient capacitance to prevent the RMS current from
causing additional upstream losses in fuses or batteries.
Other losses, including C
OUT
ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
APPLICATIO S I FOR ATIO
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Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
OUT
immediately shifts by an amount
equal to ∆I
LOAD
(ESR), where ESR is the effective series
resistance of C
OUT
. ∆I
LOAD
also begins to charge or
discharge C
OUT
generating a feedback error signal used
by the regulator to return V
OUT
to its steady-state value.
During this recovery time, V
OUT
can be monitored for
overshoot or ringing that would indicate a stability
problem. The I
TH
pin external components shown in
Figure 1 will provide adequate compensation for most
applications. For a detailed explanation of switching
control loop theory see Application Note 76.
Design Example
As a design example, take a supply with the following
specifications: V
IN
= V
REF
= 2.5V, V
EXTVCC
= 5V, V
OUT
=
1.25V ±5%, I
OUT(MAX)
= 10A, f = 250kHz. First, calculate
the timing resistor with V
ON
= V
OUT
:
R
VV V
V kHz pF V
k
ON
==Ω
125 25 07
0 7 250 10 2 5
514
.(.–.)
( . )( )( ) .
and choose the inductor for about 40% ripple current at
the maximum V
IN
:
L
V
kHz A
V
V
H=−
=µ
125
250 0 4 10
1
125
25
063
.
( )( . )( )
.
.
.
Selecting a standard value of 0.68µH results in a maximum
ripple current of:
∆=
µ
=I
V
kHz H
V
V
A
L
125
250 0 68
1
125
25
37
.
()(.)
–
.
.
.
Next, choose the synchronous MOSFET switch. Choosing
a Si4874 (R
DS(ON)
= 0.0083Ω (NOM) 0.010Ω (MAX),
θ
JA
= 40°C/W) yields a nominal sense voltage of:
V
SNS(NOM)
= (10A)(1.3)(0.0083Ω) = 108mV
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LTC3717EUH-1#TRPBF

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Manufacturer:
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Description:
Switching Voltage Regulators Power Supply for DDR in QFN
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