MAX1966 Maxim, MAX1966 Datasheet - Page 11

• Manufacturer Part Number: MAX1966
• Description: Low-Cost Voltage-Mode PWM Step-Down Controllers
• Manufacturer: Maxim

4) Select compensation capacitor C
The input capacitor (C
the current peaks drawn from the input supply. The
source impedance to the input supply determines the
value of C
input capacitance. The input capacitor must meet the
ripple current requirement (I
switching currents. The RMS input ripple current is
given by:
For optimal circuit reliability, choose a capacitor that
has less than a 10°C temperature rise at the peak rip-
ple current.
The MAX1966/MAX1967s’ step-down controller drives
two external logic-level N-channel MOSFETs. The key
selection parameters are:
1) On-resistance (R
2) Current capability of V
3) Voltage rating and maximum input voltage
Worst-case conduction losses occur at the duty factor
extremes. For the high-side MOSFET, the worst-case
power dissipation due to resistance occurs at minimum
input voltage:
The following switching loss calculation for the high-
side N-FET provides an approximation, but is no substi-
tute for evaluation:
P
P
D N SWITCHING
D N RESISTIVE
(
amp zero is equal to the complex pole frequency
LC of the inductor and output capacitor:
rent limit and efficiency
charge (Q
(
1
1
/
I
RMS
2
. High source impedance requires high
I
T
LOAD
)
)
)
______________________________________________________________________________________
C
6
DS(ON)
V
I
I
LOAD
GATE
VIN MIN
Input Capacitor Selection
Power MOSFET Selection
V
2
OUT
) reduces noise injection and
(
V
MOSFET Power Dissipation
L
OUT
L
) of both MOSFETs for cur-
(MAX1967 only) and gate
)
R
V
VIN MAX
3
C
RMS
OUT
I
V
(
LOAD
VIN
V
) imposed by the
VIN
6
)
2
so that the error
2
V
f
OUT
OSC
R
Low-Cost Voltage Mode PWM
DS ON
(
C
RSS
)
where C
and I
(1A typical). For the low-side N-FET (N2), the worst-
case power dissipation occurs at maximum input volt-
age:
The low-side MOSFET on-resistance sets the
MAX1966/MAX1967 current limit. See the Setting the
Current Limit section for information on selecting low-
side MOSFET R
less, it is often possible to combine the high-side and
low-side MOSFETs into a single package (usually an 8-
pin SO) as indicated in Table 1. For higher output appli-
cations, or those where efficiency is more important,
separate FETs are usually preferred.
The MAX1966/MAX1967 are extremely versatile con-
trollers that can be used in a variety of applications
where high efficiency, high output power, and opti-
mized cost are important. One alternate connection,
shown in Figure 5, is useful when a low-voltage supply
is to be stepped down to an even lower voltage at high
current. If an additional bias supply is available, it can
supply gate drive separately from the input power rail.
This can either improve efficiency, or allow lower cost
5V logic-level MOSFETs to be used in place of 3V
MOSFETs.
Figure 5. Low Input Voltage Step-Down with Extra Bias Supply
for Gate Drive
5V TO 28V FOR
GATE BIAS
Step-Down Controller
GATE
P
D N
RSS
(
2
is the peak gate-drive source/sink current
)
is the reverse transfer capacitance of N1
VIN
COMP/EN
GND
MAX1967
Very-Low-Voltage Applications
DSON
1
V
V
VCC
OUT
BST
VIN
DH
DL
VL
LX
FB
. For designs supplying 5A or
R2
I
LOAD
R1
2
R
DS ON
(
3.3V
INPUT
)
V
OUT
11