Datasheet

tance as possible. For continuous inductor current, the
power loss in the inductor resistance (PLR) is approx-
imated by:
2
LOAD
LR L
MAX
I
P ~R x
I D


−

where R
L
is the inductor series resistance.
Once the peak inductor current is calculated, the cur-
rent sense resistor, R
CS
, is determined by:
R
CS
= 85mV / I
LPEAK
For high peak inductor currents (>1A), Kelvin-sensing
connections should be used to connect CS and PGND
to RCS. Connect PGND and GND together at the ground
side of R
CS
. A lowpass filter between R
CS
and CS may
be required to prevent switching noise from tripping the
current-sense comparator at heavy loads. Connect a
100W resistor between CS and the high side of R
CS
, and
connect a 1000pF capacitor between CS and GND.
Checking Slope-Compensation Stability
In a current-mode regulator, the cycle-by-cycle stability
is dependent on slope compensation to prevent subhar-
monic oscillation at duty cycles greater than 50%. For the
MAX1846/MAX1847, the internal slope compensation
is optimized for a minimum inductor value (L
MIN
) with
respect to duty cycle. For duty cycles greater then 50%,
check stability by calculating LMIN using the following
equation:
( )
( ) ( )
MIN IN(MIN) CS S
MAX MAX
L V xR /M
x 2xD 1 / 1 D
−−

=



where V
IN(MIN)
is the minimum expected input voltage,
M
s
is the Slope Compensation Ramp (41 mV/µs) and
D
MAX
is the maximum expected duty cycle. If L
MIN
is
larger than L, increase the value of L to the next standard
value that is larger than L
MIN
to ensure slope compensa-
tion stability.
Choosing the Inductor Core
Choosing the most cost-effective inductor usually requires
optimizing the field and flux with size. With higher output
voltages the inductor may require many turns, and this
can drive the cost up. Choosing an inductor value at L
MIN
can provide a good solution if discontinuous inductor cur-
rent can be tolerated. Powdered iron cores can provide
the most economical solution but are larger in size than
ferrite.
Power MOSFET Selection
The MAX1846/MAX1847 drive a wide variety of P-channel
power MOSFETs (PFETs). The best performance, espe-
cially with input voltages below 5V, is achieved with
low-threshold PFETs that specify on-resistance with
a gate-to-source voltage (V
GS
) of 2.7V or less. When
selecting a PFET, key parameters include:
● Total gate charge (Q
G
)
● Reverse transfer capacitance (C
RSS
)
● On-resistance (
RDS(ON)
)
● Maximum drain-to-source voltage (V
DS(MAX)
)
● Minimum threshold voltage (V
TH(MIN)
)
At high-switching rates, dynamic characteristics (para-
meters 1 and 2 above) that predict switching losses
may have more impact on efficiency than R
DS(ON)
,
which predicts DC losses. Q
G
includes all capacitance
associated with charging the gate. In addition, this
parameter helps predict the current needed to drive the
gate at the selected operating frequency. The power
MOSFET in an inverting converter must have a high
enough voltage rating to handle the input voltage plus
the magnitude of the output voltage and any spikes
induced by leakage inductance and ringing.
An RC snubber circuit across the drain to ground might be
required to reduce the peak ringing and noise.
Choose R
DS(ON)(MAX)
specified at V
GS
< V
IN(MIN)
to be
one to two times R
CS
. Verify that V
IN(MAX)
< V
GS(MAX)
and V
DS(MAX)
> V
IN(MAX)
- V
OUT
+ V
D
. Choose the rise-
and fall-times (t
R
, t
F
) to be less than 50ns.
Output Capacitor Selection
The output capacitor (C
OUT
) does all the filtering in an
inverting converter. The output ripple is created by the
variations in the charge stored in the output capacitor with
each pulse and the voltage drop across the capacitor’s
equivalent series resistance (ESR) caused by the current
into and out of the capacitor. There are two properties of
the output capacitor that affect ripple voltage: the capac-
itance value, and the capacitor’s ESR. The output ripple
due to the output capacitor’s value is given by:
V
RIPPLE-C
= (I
LOAD
× D
MAX
× T
OSC
) / C
OUT
The output ripple due to the output capacitor’s ESR is
given by:
V
RIPPLE-R
= I
LPP
× R
ESR
These two ripple voltages are additive and the total output
ripple is:
V
RIPPLE-T
= V
RIPPLE-C
+ V
RIPPLE-R
www.maximintegrated.com
Maxim Integrated │ 14
MAX1846–MAX1847 High-Efciency, Current-Mode,
Inverting PWM Controller