LTC1624
U
W U U
APPLICATIONS INFORMATION
V
V
+ V
(
)
V + V
+ V
D
IN
(
OUT D
)
IN
OUT
∆I
=
where: I
=I
L P-P
(
SW MAX
OUT MAX
)
(
)
(
)
V
200kHz L V + V
+ V
D
IN
IN
OUT
Specify the maximum inductor current to safely handle
IL(PEAK). Make sure the inductor’s saturation current rat-
ing (current when inductance begins to fall) exceeds the
δ is the temperature dependency of RDS(ON) and k is a
constant inversely related to the gate drive current. The
maximum switch current occurs at VIN(MIN) and the peak
switch current is ISW(MAX) + ∆IL/2. The maximum voltage
maximum current rating set by RSENSE
.
across the switch is VIN(MAX) + VOUT
.
Positive-to-Negative Converter: RSENSE Selection for
Maximum Output Current
MOSFETs have I2R losses plus the PMAIN equation
includes an additional term for transition losses that are
highest at high total input plus output voltages. For
( VOUT + VIN) < 20V the high current efficiency generally
improves with larger MOSFETs, while for ( VOUT + VIN)
> 20V the transition losses rapidly increase to the point
that the use of a higher RDS(ON) device with lower CRSS
actual provides higher efficiency. For additional informa-
tion refer to the Step-Down Converter: Power MOSFET
Selection in the Applications Information section.
RSENSE is chosen based on the required output current.
Remember the LTC1624 current comparator has a maxi-
mum threshold of 160mV/RSENSE. The current compara-
torthresholdsetsthepeakoftheinductorcurrent,yielding
a maximum average output current IOUT(MAX) equal to
IL(PEAK) less half the peak-to-peak ripple current with the
remainder divided by the duty cycle.
Allowing a margin for variations in the LTC1624 (without
consideringvariationinRSENSE)andassuming30%ripple
current in the inductor, yields:
Positive-to-Negative Converter: Inductor Selection
For most applications the inductor will fall in the range of
10µH to 100µH. Higher values reduce the input and output
ripple voltage (although not as much as step-down con-
verters) and also reduce core loss. Lower inductor values
are chosen to reduce physical size and improve transient
response but do increase output ripple.
V
IN MIN
(
)
100mV
R
=
SENSE
I
V
+ V
+ V
D
OUT MAX
IN MIN
OUT
Positive-to-Negative Converter: Output Diode
The output diode conducts current only during the switch
off-time. Peak reverse voltage for positive-to-negative
converters is equal to VOUT + VIN. Average forward
current in normal operation is equal to ID(PEAK) – ∆IL/2.
Peak diode current (occurring at VIN(MIN)) is:
Like the boost converter, the input current of the positive-
to-negative converter is calculated at full load current.
Peak inductor current can be significantly higher than
output current, especially with smaller inductors (with
high ∆IL values). The following formula assumes continu-
ous mode operation and calculates maximum peak induc-
tor current at minimum VIN:
V
+V
D
(
OUT
)
∆I
2
L
I
=I
+1 +
D PEAK
OUT MAX
(
)
(
)
V
IN
V + V
+ V
D
∆I
2
IN
OUT
L
I
=I
+
L PEAK
OUT MAX
(
)
(
)
V
IN
Positive-to-Negative Converter: Input and
Output Capacitors
The ripple current in the inductor (∆IL) is typically 20% to
50% of the peak inductor current occuring at VIN(MIN) and
IOUT(MAX) to minimize output ripple. Maximum ∆IL occurs
at minimum VIN.
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
output ripple voltage. Both input and output capacitors
need to be sized to handle the ripple current safely.
19
Linear [ Linear ]
