MAX1624 Datasheet, PDF (16/24 Pages) Maxim Integrated Products – High-Speed Step-Down Controllers with Synchronous Rectification for CPU Power

High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
GlitchCatcher
Current-Boost Driver (MAX1624)
Drivers for an optional current-boost circuit are includ-
ed in the MAX1624 to improve transient response.
Some dynamically clocked CPUs switch computational
blocks on and off as needed to reduce power con-
sumption, and can generate load steps of several
amperes in a few tens of nanoseconds. The current-
boost circuit is intended to improve transient response
to such load steps by bypassing the inductor’s lowpass
filter operation. When the output drops out of regulation
by more than ±1.5% to ±2.5%, the P-channel or
N-channel switches turn on and force the output back
into regulation. The MOSFET drivers’ response time is
typically 75ns, and their minimum on-time is typically
100ns.
Current Sense
and Overload Current Limiting
The current-sense circuit resets the main PWM latch
and turns off the high-side MOSFET switch whenever
the voltage difference between CSH and CSL from cur-
rent through the sense resistor (R1) exceeds the peak
current limit (100mV typical).
Current-mode control offers a practical level of over-
load protection in response to many fault conditions.
During normal operation, maximum output current is
enforced by the peak current limit. If the output is short-
ed directly to GND through a low-resistance path, the
current-sense comparator may be unable to enforce a
current limit. Under such conditions, circuit parasitics
such as MOSFET R_DS(ON) typically limit the short-
circuit current 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 (Figure 6). The filter should be
designed with a time constant of around 200ns.
Resistors in the 20Ω to 100Ω range are recommended
for R7 and R8. Connect the filter capacitors C11 and
C12 from V_CC to CSH and CSL, respectively.
Values of 39Ω and 4.7nF 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.
Internal Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit at start-up to reduce input surge currents. In
the MAX1624/MAX1625, an internal DAC raises the cur-
rent-limit threshold from 0V to 100mV in four steps
(25mV, 50mV, 75mV, and 100mV) over the span of
1536 oscillator cycles.
C9
0.1µF
C7
4.7µF
V_CC
CSH
MAX1624
MAX1625
CSL
R6
100Ω
C11
4.7nF
C12
R7
4.7nF
39Ω
R8
39Ω
V_IN
C1
R1
N1
Figure 6. Current-Sense Filter
__________________Design Procedure
Setting the Output Voltage
MAX1624
Select the output voltage using the D0–D4 pins. The
MAX1624 uses an internal 5-bit DAC as a feedback-
resistor voltage divider. The output voltage can be digi-
tally set in 100mV increments from 1.1V to 3.5V using
the D0–D4 inputs (Table 4).
D0–D4 are logic inputs and accept both TTL and
CMOS voltage levels. The MAX1624 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 MAX1624 DAC codes were designed for compati-
bility with Intel specifications for output voltages
between 3.5V and 2.1V. Codes 10000 through 11110
are compatible with Intel specifications, while codes
00000 through 01111 are not. Codes 11111 and 01111
turn off the buck controller, placing the IC in a low-
current mode (0.2mA typical). For compatibility with
Intel codes for output voltages below 2.1V, see the
MAX1638/MAX1639 data sheet.
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