Rainbow Electronics MAX1845 - Manual (Page 10)

MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
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Pin Description (continued)
PIN NAME FUNCTION
22 V CC
Analog Supply Input. Connect to system supply voltage, 4.5V to 5.5V, with a 20 Ω series resistor.
Bypass to GND with a 1µF capacitor.
23 GND Ground. Combined analog and power ground. Serves as negative input for CS_ amplifiers.
24 DL1 Low-Side Gate-Driver Output for OUT1. DL1 swings from GND to V DD .
25 BST1
Boost Flying Capacitor Connection for OUT1. Connect to an external capacitor and diode
according to the standard application circuit in Figure 1. See MOSFET Gate Drivers (DH_, DL_)
section
26 DH1 High-Side Gate Driver Output for OUT1. Swings from LX1 to BST1.
27 LX1
External Inductor Connection for OUT1. Connect to the switched side of the inductor. LX1 serves as
the internal lower supply voltage rail for the DH1 high-side gate driver.
28 CS1
Current-Sense Input for OUT1. CS1 is the input to the current-limiting circuitry for valley current
limiting. For lowest cost and highest efficiency, connect to LX1. For highest accuracy, connect to a
sense resistor. See the Current-Limit Circuit (ILIM_) section.
Standard Application Circuit
The standard application circuit (Figure 1) generates a
1.8V and a 2.5V rail for general-purpose use in note-
book computers.
See Table 1 for component selections. Table 2 lists
component manufacturers.
Detailed Description
The MAX1845 buck controller is designed for low-volt-
age power supplies for notebook computers. Maxim’s
proprietary Quick-PWM pulse-width modulator in the
MAX1845 (Figure 2) is specifically designed for han-
dling fast load steps while maintaining a relatively con-
stant operating frequency and inductor operating point
over a wide range of input voltages. The Quick-PWM
architecture circumvents the poor load-transient timing
problems of fixed-frequency current-mode PWMs while
avoiding the problems caused by widely varying
switching frequencies in conventional constant-on-time
and constant-off-time PWM schemes.
5V Bias Supply (V CC and V DD )
The MAX1845 requires an external 5V bias supply in
addition to the battery. Typically, this 5V bias supply is
the notebook’s 95% efficient 5V system supply.
Keeping the bias supply external to the IC improves
efficiency and eliminates the cost associated with the
5V linear regulator that would otherwise be needed to
supply the PWM circuit and gate drivers. If stand-alone
capability is needed, the 5V supply can be generated
with an external linear regulator such as the MAX1615.
The power input and 5V bias inputs can be connected
together if the input source is a fixed 4.5V to 5.5V sup-
ply. If the 5V bias supply is powered up prior to the bat-
tery supply, the enable signal (ON1, ON2) must be
delayed until the battery voltage is present to ensure
startup. The 5V bias supply must provide V
CC and
gate-drive power, so the maximum current drawn is:
I BIAS = I CC + f (Q G1 + Q G2 ) = 5mA to 30mA (typ)
where I CC is 1mA typical, f is the switching frequency,
and Q G1 and Q G2 are the MOSFET data sheet total
gate-charge specification limits at V GS = 5V.
Free-Running, Constant-On-Time PWM
Controller with Input Feed-Forward
The Quick-PWM control architecture is a pseudo-fixed-
frequency, constant-on-time current-mode type with
voltage feed-forward (Figure 3). This architecture relies
on the output filter capacitor’s effective series resis-
tance (ESR) to act as a current-sense resistor, so the
output ripple voltage provides the PWM ramp signal.
The control algorithm is simple: the high-side switch
on-time is determined solely by a one-shot whose pulse
width is inversely proportional to input voltage and
directly proportional to output voltage. Another one-shot
sets a minimum off-time (400ns typ). The on-time one-
shot is triggered if the error comparator is low, the low-
side switch current is below the current-limit threshold,
and the minimum off-time one-shot has timed out
(Table 3).