Rainbow Electronics MAX1638 - Manual (Page 11)

MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
______________________________________________________________________________________ 11
High-Side Current Sensing
The common-mode input range of the current-sense
inputs (CSH and CSL) extends to V
CC , so it is possible
to configure the circuit with the current-sense resistor
on the input side rather than on the load side (Figure 5).
This configuration improves efficiency by reducing the
power dissipation in the sense resistor according to the
duty ratio.
In the high-side configuration, if the output is shorted
directly to GND through a low-resistance path, the cur-
rent-sense comparator may be unable to enforce a cur-
rent limit. Under such conditions, circuit parasitics such
as MOSFET R
DS(ON) typically limit the short-circuit cur-
rent to a value around the peak-current-limit setting.
Attach a lowpass-filter network between the current-
sense pins and resistor to reduce high-frequency com-
mon-mode noise. The filter should be designed with a
time constant of around one-fifth of the on-time (130ns
at 600kHz, for example). Resistors in the 20
Ω
to 100
Ω
range are recommended for R7 and R8. Connect the
filter capacitors C11 and C12 from V
CC to CSH and
CSL, respectively.
Values of 39
Ω
and 3.3nF are suitable for many designs.
Place the current-sense filter network close to the IC,
within 0.1 in (2.5mm) of the CSH and CSL pins.
Overvoltage Protection
When the output exceeds the set voltage, the synchro-
nous rectifier (N2) is driven high (and N1 is driven low).
This causes the inductor to quickly dissipate any stored
energy and force the fault current to flow to ground.
Current is limited by the source impedance and para-
sitic resistance of the current path, so a fuse is required
in series with the +5V input to protect against low-
impedance faults, such as a shorted high-side MOS-
FET. Otherwise, the low-side MOSFET will eventually
fail. DL will go low if the input voltage drops below the
undervoltage lockout point.
Internal Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit at start-up to reduce input surge currents. An
internal DAC raises the current-limit threshold from 0V
to 100mV in four steps (25mV, 50mV, 75mV, and
100mV) over the span of 1536 oscillator cycles.
__________________Design Procedure
Setting the Output Voltage
Select the output voltage using the D0–D4 pins. The
MAX1638 uses an internal 5-bit DAC as a feedback-
resistor voltage divider. The output voltage can be digi-
tally set from 1.3V to 3.5V using the D0–D4 inputs
(Table 2).
D0–D4 are logic inputs and accept both TTL and
CMOS voltage levels. The MAX1638 has both FB and
AGND inputs, allowing a Kelvin connection for remote
voltage and ground sensing to eliminate the effects of
trace resistance on the feedback voltage. (See
PC
Board Layout Considerations for further details.) FB
input current is 0.1µA (max).
The MAX1638 DAC codes (D0–D4) were designed for
compatibility with the Intel VRM 8.2 specification for
output voltages between 1.8V (code 00101) and 3.5V
(code 10000). Codes 00110 to 01111 have also been
designed for 50mV increments, allowing set voltages
down to 1.300V. Code 11111 turns off the buck controller,
placing the IC in a shutdown mode (0.2mA typical).
Choosing the
Error-Amplifier Gain
Set the error-amplifier gain to match the voltage-preci-
sion requirements of the CPU used. The MAX1638’s
loop-gain control input (LG) allows trade-offs in DC/AC
voltage accuracy versus output filter capacitor require-
ments. AC load regulation can be set to 0.5%, 1%,
or 2% by connecting LG as shown in Table 3.
DC load regulation is typically 10 times better than AC
load regulation, and is determined by the gain set by
the LG pin.
0
20
10
50
40
30
60
70
100
90
80
20 30 10 0 40 50 60 70 80 90 100
V
FB
(%)
I LIM
(%)
Figure 4. Foldback Current Limit