Rainbow Electronics MAX1847 User Manual
Page 14
MAX1846/MAX1847
High-Efficiency, Current-Mode,
Inverting PWM Controller
14
______________________________________________________________________________________
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 R
CS
. 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 100
Ω 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 sub-
harmonic oscillation at duty cycles greater than 50%.
For the MAX1846/MAX1847, the internal slope compen-
sation 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 fol-
lowing equation:
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 stan-
dard value that is larger than L
MIN
to ensure slope
compensation stability.
Power MOSFET Selection
The MAX1846/MAX1847 drive a wide variety of P-chan-
nel power MOSFETs (PFETs). The best performance,
especially 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:
1)
Total gate charge (Q
G
)
2)
Reverse transfer capacitance (C
RSS
)
3) On-resistance
(R
DS(ON)
)
4)
Maximum drain-to-source voltage (V
DS(MAX)
)
5)
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.
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 capacitance 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 out-
put ripple is:
V
RIPPLE-T
= V
RIPPLE-C
+ V
RIPPLE-R
The ESR-induced ripple usually dominates this last
equation, so typically output capacitor selection is
based mostly upon the capacitor’s ESR, voltage rating,
and ripple current rating. Use the following formula to
determine the maximum ESR for a desired output ripple
voltage (V
RIPPLE-D
):
R
ESR
= V
RIPPLE-D
/ I
LPP
Select a capacitor with ESR rating less than R
ESR
. The
value of this capacitor is highly dependent on dielectric
type, package size, and voltage rating. In general, when
choosing a capacitor, it is recommended to use low-ESR
capacitor types such as ceramic, organic, or tantalum
capacitors. Ensure that the selected capacitor has suffi-
cient margin to safely handle the maximum ripple current
(I
LPP
) and the maximum output voltage.
Choosing Compensation Components
The MAX1846/MAX1847 are externally loop-compen-
sated devices. This provides flexibility in designs to
accommodate a variety of applications. Proper com-
L
V
R
M
D
D
MIN
IN MIN
CS
S
MAX
MAX
=
Ч
(
)
[
]
Ч
Ч
(
) (
)
[
]
−
−
(
)
/
/
2
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