MAX1711 Maxim, MAX1711 Datasheet - Page 20

• Manufacturer Part Number: MAX1711
• Description: High-Speed / Digitally Adjusted Step-Down Controllers for Notebook CPUs
• Manufacturer: Maxim

High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
Figure 7. All-Ceramic-Capacitor Application
where C
and I
typical).
For the low-side MOSFET, Q2, the worst-case power dis-
sipation always occurs at maximum battery voltage:
The absolute worst case for MOSFET power dissipation
occurs under heavy overloads that are greater than
I
20
Table 5. Approximate K-Factors Errors
SETTING
LOAD(MAX)
PD(Q2) = (1 - V
(kHz)
TON
PD switching
200
300
400
550
______________________________________________________________________________________
GATE
(
RSS
FACTOR
is the peak gate-drive source/sink current (1A
but are not quite high enough to exceed the
(µs-V)
is the reverse transfer capacitance of Q1
3.3
2.5
1.8
K
5
)
OUT
C
* FOR HIGHER MINIMUM INPUT VOLTAGE,
*
RSS
LESS OUTPUT CAPACITANCE IS REQUIRED.
V
/ V
APPROXIMATE
IN
ERROR (%)
= 7V TO 24V*
BATT(MAX)
K-FACTOR
V
BATT MAX
±12.5
±12.5
±10
±10
I
INPUTS
GATE
(
DAC
ON/OFF
0.22 F
) · I
470pF
)
0.1 F
2
LOAD 2
f I
AT V
SHDN
SKIP
D0
D1
D2
D3
D4
REF
CC
TON
LOAD
V+
MIN V
· R
OUT
2.6
2.9
3.2
3.6
(V)
MAX1711
DS(ON)
BATT
V
CC
= 2V
20
GNDS
PGND
GND
V
BST
FBS
DH
LX
DL
DD
FB
current limit and cause the fault latch to trip. To protect
against this possibility, you must “overdesign” the circuit
to tolerate I
where I
by the current-limit circuit, including threshold tolerance
and on-resistance variation. This means that the
MOSFETs must be very well heatsinked. If short-circuit
protection without overload protection is enough, a nor-
mal I
stresses.
Choose a Schottky diode D1 having a forward voltage
low enough to prevent the Q2 MOSFET body diode from
turning on during the dead time. As a general rule, a
diode having a DC current rating equal to 1/3 of the load
current is sufficient. This diode is optional, and if efficien-
cy isn’t critical it can be removed.
The output voltage adjust range for continuous-conduc-
tion operation is restricted by the non-adjustable 500ns
(max) minimum off-time one-shot. For best dropout per-
formance, use the slowest (200kHz) on-time setting.
When working with low input voltages, the duty-factor
limit must be calculated using worst-case values for on
and off-times. Manufacturing tolerances and internal
5
1nF
LOAD
LIMIT(HIGH)
0.1 F
+5V
value can be used for calculating component
LOAD
1 F
C1 = 4 x 4.7 F/25V TAIYO YUDEN (TMK325BJ475K)
C2 = 6 x 47 F/10V TAIYO YUDEN (LMK550BJ476KM)
R1 + R2 = 5m MINIMUM OF PCB TRACE RESISTANCE (TOTAL)
1k
1k
1k
Q1
Q2
= I
0.5 H
is the maximum valley current allowed
LIMIT(HIGH)
C1
Application Issues
R1
R2
Dropout Performance
+ (LIR / 2) · I
C2
1.6V AT 7A
CPU
LOAD(MAX)
,