Datasheet

LT3988
10
3988f
at frequencies above f
MAX1
. It will continue to regulate but
with increased inductor current and increased output ripple.
f
MAX2
is the frequency at which the maximum duty cycle
is exceeded. If there is sufficient charge on the BOOST
capacitor, the regulator will skip OFF periods to increase
the overall duty cycle at frequencies above f
MAX2
. Note
that the restriction on the operating input voltage refers
to steady-state limits to keep the output in regulation;
the circuit will tolerate input voltage transients up to the
absolute maximum rating.
Switching Frequency
Once the upper and lower bounds for the switching
frequency are found from the duty cycle requirements,
the frequency may be set within those bounds. Lower
frequencies result in lower switching losses, but require
larger inductors and capacitors. The user must decide
the best trade-off.
The switching frequency is set by a resistor connected from
the RT pin to ground, or by forcing a clock signal into the
SYNC pin. The LT3988 applies a voltage across this resistor
and uses the current to set the oscillator speed. The R
T
resistor value for a given switching frequency is given by:
R
T
=
1.31
f
2
+
46.56
f
– 7.322
250kHz ≤ f ≤ 2.5MHz
where f is in MHz and R
T
is in kΩ.
The frequency sync signal will support V
IH
logic levels from
1.5V to 5V CMOS or TTL. The duty cycle is not important,
but it needs a minimum on time of 100ns and a minimum
off time of 100ns. R
T
should be set to provide a frequency
within ±25% of the final sync frequency.
The slope recovery circuit sets the slope compensation
to the appropriate value for the synchronized frequency.
Choose the inductor value based on the lowest potential
switching frequency.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L =
V
OUT
+ V
F
0.6A • f
where V
F
is the voltage drop of the catch diode (~0.4V) and f
is in MHz. The inductor’s RMS current rating must be greater
than the maximum load current and its saturation current
Table 1. Inductors
MFG URL PART SERIES INDUCTANCE RANGE (µH) SIZE (mm) (L × W × H)
Coilcraft http://www.coilcraft.com XPL7030
XFL4020
XAL50XX
0.13 to 22
1 to 4.7
0.16 to 22
7 × 7 × 3
4 × 4 × 2.15
5.28 × 5.48 × 5.1
Cooper http://www.cooperbussmann.com DRA74
DR1040
0.33 to 1000
1.5 to 330
7.6 × 7.6 × 4.35
10.5 × 10.3 × 4
CWS http://www.coilws.com SP-0703
SP-0704
SB-1004
3 to 100
2.2 to 100
10 to 1500
7 × 7 × 3
7 × 7 × 4
10.1 × 10.1 × 4.5
Murata http://www.murata.com LQH55D
LQH6PP
LQH88P
0.12 to 10000
1 to 100
1 to 100
5 × 5.7 × 4.7
6 × 6 × 4.3
8 × 8 × 3.8
Sumida http://www.sumida.com CDMC6D28
CDEIR8D38F
0.2 to 4.7
4 to 22
7.25 × 6.7 × 3
8.5 × 8.3 × 4
Toko http://www.toko.co.jp DS84LCB
FDV0620
1 to 100
0.2 to 4.7
8.4 × 8.3 × 4
6.7 × 7.4 × 2
Vishay http://www.vishay.com IHLP-2020AB-11
IHLP-2020BZ-11
IHLP-2525CZ-11
0.1 to 4.7
0.1 to 10
1 to 22
5.49 × 5.18 × 1.2
5.49 × 5.18 × 2
6.86 × 6.47 × 3
Würth http://www.we-online.de WE-PD2-S
WE-PD-M
WE-PD2-XL
1 to 68
1 to 1000
10 to 820
4 × 4.5 × 3.2
7.3 × 7.3 × 4.5
9 × 10 × 5.4
applicaTions inForMaTion