ISL85402(2011) データシートの表示（PDF） - Intersil

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ISL85402
(Rev.:2011)

2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

Intersil 

ISL85402

Where VFTH is the desired falling threshold on boost input

voltage to turn on the boost, 3µA is the hysteresis current, and

0.8V is the reference voltage to be compared with.

Note the boost start-up threshold has to be selected in a way that

the buck is operating working well and kept in close loop

regulation before boost start-up. Otherwise, large in-rush current

at boost start-up could occur at boost input due to the buck open

loop saturation.

Similarly, a resistor divider from boost output voltage to AUXVCC pin

is used to detect the boost output voltage. When the voltage on

AUXVCC pin is below 0.8V, the boost PWM is enabled with a fixed

500µs soft-start, and a 3µA sinking current is enabled at AUXVCC

pin for hysteresis purposes. When the voltage on the AUXVCC pin

recovers to be above 0.8V, the boost PWM is disabled immediately.

Use Equation 3 to calculate the upper resistor RUP (R3 in Figure 25)

according to a desired hysteresis VHY at boost output voltage. Use

Equation 4 to calculate the lower resistor RLOW (R4 in Figure 25)

according to a desired boost enable threshold at boost output.

Assuming VBAT is the boost input voltage, VOUTBST is the boost

output voltage and VOUT is the buck output voltage, the steady state

transfer function are:

VOUTBST

=

-------1----------

1–D

⋅

VBA

T

(EQ. 5)

VOUT

=

D ⋅ VOUTBST

=

-------D----------

1–D

⋅

VB

A

T

(EQ. 6)

the boost output voltage that is 5.2V (Equation 7), meaning the

VIN pin (buck input) still sees 5.2V to keep the IC working.

Note in the previous mentioned case, the boost input current could

be high because the input voltage is very low

(VIN*IIN = VOUT*IOUT*Efficiency). If the design is to achieve the low

input operation with full load, the inductor and MOSFET have to be

selected to be with enough current ratings to handle the high current

appearing at boost input. The boost inductor current are the same

with the boost input current, which can be estimated as Equation 8,

where POUT is the output power, VBAT is the boost input voltage, EFF

is the estimated efficiency of the whole boost and buck stages.

ILIN = -V----B-----AP-----T-O-----⋅U----E-T----F-----F--

(EQ. 8)

Based on the same concerns of boost input current, the IC should be

disabled before the boost input voltage rise above a certain level.

PFM is not available in boost mode.

Oscillator and Synchronization

The oscillator has a default frequency of 500kHz with FS pin

connected to VCC, or ground, or floating. The frequency can be

programmed to any frequency between 200kHz and 2.2MHz with a

resistor from FS pin to GND.

RFS[kΩ] = -1----4-----5-----0-----0-----0--F-----–S-----[-1---k--6---H----⋅--z--F--]---S-----[---k-----H------z----]-

(EQ. 9)

From Equations 5 and 6, Equation 7 can be derived to estimate the

steady state boost output voltage as function of VBAT and VOUT:

VOUTBST = VBAT + VOUT

(EQ. 7)

After the IC starts up, the boost buck converters can keep working

when the battery voltage drops extremely low because the IC’s bias

(VCC) LDO is powered by the boost output. For example, a 3.3V

output application, battery drops to 2V, VIN pin voltage is powered by

BATTERY

+

VOUT_BST

+

R1

EXT_BOOST

R2

R3

AUXVCC

R4

14

0.8V

I_HYS = 3µA

0.8V

I_HYS = 3µA

LOGIC

PWM

LGATE

DRIVE

LGATE

FIGURE 25. BOOST CONVERTER CONTROL

FN7640.0

September 29, 2011

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